GB2390605A

GB2390605A - Fibre-reinforced epoxy resin composition

Info

Publication number: GB2390605A
Application number: GB0316285A
Authority: GB
Inventors: Martin Simmons; John Cawse
Original assignee: Hexcel Composites Ltd
Current assignee: Hexcel Composites Ltd
Priority date: 2002-07-12
Filing date: 2003-07-11
Publication date: 2004-01-14
Also published as: GB0216170D0; GB0316285D0

Abstract

A resin composition comprises a fibrous reinforcement impregnated with a resin matrix, said resin matrix further comprises: <SL> <LI>(i) a resin monomer having the general structure formula I shown below: <EMI ID=1.1 HE=32 WI=63 LX=925 LY=958 TI=CF> in which: ```L is a linking group which may comprise aliphatic, aromatic or alicyclic units; ```R<1> is H, CN or C Ü -C Å alkyl; ```R<2> is H or together with R<9> forms part of a saturated ring of five or six carbon atoms; ```R<9> is H or together with R<2> forms part of a saturated ring of five or six carbon atoms; <LI>(ii) a curing agent for the epoxy group; and <LI>(iii) a curing agent for the vinyl ester group. </SL> Also claimed are processes for the preparation of a prepreg composition, a partially cured form of a prepreg composition, and a fully cured fibre-reinforced composite.

Description

À. "I; ' À 2390605 À. ee: i À - Resin Compositions The present invention

relates to resin compositions and, more particularly but not exclusively, to resin compositions suitable for use in the preparation of fibre-

reinforced resin composites and processes for preparing the same.

5 Fibre-reinforced resin compositions are widely used to form structural components for use in a range of different applications such as, for example, the manufacture of components for use in the aerospace, transport, electronics, building and leisure industries. These fibrereinforced resin composites are generally supplied in the form of a "prepreg" composition, although these compositions may also be 10 supplied in alternative forms such as an adhesive or a component of a resin film infusion material.

A prepreg composition generally comprises a fibrous reinforcement phase impregnated with a resin matrix comprising one or more liquid resins and one or more curing agents. A typical prepreg composition may also include additional constituents 15 such as tougheners, flame retardants, and fillers. The prepreg composition can be moulded into the required configuration by the user and then cured by heating the composition to form the final fibre-reinforced composite structure.

Known prepreg compositions invariably possess a certain level of tack (stickiness). Although a certain level of tack is needed in most applications of 20 prepreg, there are situations in which an excessive tackiness can make the handling

À:e tee. À.;:: À:e:e and manipulation of the prepreg composition by the user extremely difficult and, for this reason, there is a desire in these instances for the provision of prepreg compositions that possesses little or no tack. For example, in applications where prepreg is being handled and cut automatically, it can be undesirable for the prepreg to F 5 be excessively tacky because tacky deposits may accumulate on the blades used for cutting the prepreg. This can in turn lead to further difficulties in cutting subsequent pieces from the prepreg, or to dust and grit particles adhering to the cutting machinery. For epoxy resin prepreg compositions, it has been found that the addition of 10 other epoxy resins to the composition successfully reduces the degree of tack by increasing the viscosity of the composition. However, this approach cannot be used for prepreg compositions which need to be cured at low temperatures, for example 100 C or less, because the composition will not be sufficiently mobile at this temperature and elevating the temperature further causes the composition to cure 15 prematurely.

An alternative approach to reduce the tack of epoxy resin prepreg compositions is to use a process known as B-staging. B-staging is a process in which an epoxy resin is reacted with a limited amount of a curing agent to allow the resin to partially cure, but without allowing a full cure to occur. It has been found that B 20 staging can successfully reduce the tack of the prepreg composition to an acceptable

l c:: be: be* c': . . : ce. À:::: e: À À. À À

level, but results in a composition that cannot then be converted into the final cured form at a suitably low temperature or within a short period of time.

So-called vinyl ester based resins are also widely used in composite formulations, particularly as liquid resins for applications such as moulding or 5 infusion into glass fibre reinforced composites. Vinyl ester resins are also known as epoxy acrylates and epoxy methacrylates and most commonly contain the unsaturated acrylic or methacrylic functionality. These resins possess a number of favourable characteristics such as the ability to undergo a rapid cure at low temperatures and the ability to form a final cured resin having good mechanical properties. However, due 10 to the liquid nature of these resins, a wet lay-up procedure is required which renders the provision of a prepreg composition having little or no tack impossible to achieve.

Furthermore, it is not possible to use B-staging to prepare a vinyl ester based prepreg composition having little or no tack unless the resin is initially converted into the form of a gel.

15 Alternative attempts to provide a dry tack vinyl ester resin composition by the addition of fillers, low molecular weight polymers, thickening agents (such as magnesium oxide and magnesium hydroxide) and waxes to the composition have succeeded in reducing the tack of the prepreg composition, but the inclusion of these additives was found to adversely affect the glass transition temperature (Tg) and the 20 mechanical properties of the final cured composition.

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A, The preparation of multi-component formulations that contain mixtures of both epoxy and vinyl ester resins has also failed to produce a prepreg composition which possesses the desired properties. Although the reasons for this may be diverse,: one deficiency commonly encountered is a lack of compatibility between the two I 5 resin chemistries leading to phase separation during the cure process. Cured resins prepared in this way exhibit poor mechanical properties.

An object of the present invention is, therefore, to provide a prepreg composition, or a resin suitable for the preparation thereof, that possesses little or no tack and that can also be cured at a conveniently low temperature to yield the final 10 cured composite.

According to a first aspect of the present invention there is provided a resin composition comprising a fibrous reinforcement impregnated with a resin matrix, said resin matrix further comprising: (i) a resin monomer having the general structural formula I shown below R CHR9 L-O I I

15 R1 in which:

À re.e /. À, , À

À a À À À À

S L is a linking group which may comprise aliphatic, aromatic or alicyclic units; R' is H. CN or C,-C5 alkyl; R2 is H or together with R9 forms part of a saturated ring of five 5 or six carbon atoms; R9 is H or together with R2 forms part of a saturated ring of five I or six carbon atoms; (ii) a curing agent for the epoxy group; and (iii) a curing agent for the vinyl ester group.

10The prepreg compositions of the invention can be readily converted into a partially cured form. The expression "partially cured form" is used herein to denote compositions of the invention that are stable over extended periods of time, possess the desired property of having little or no tack and can be readily handled and manipulated into the required shape and configuration by a user. Furthermore, the I 5 partially cured form of the prepreg can be readily converted into a fully cured fibre-

reinforced composite by heating the prepreg at a conveniently low temperature in the range of 60 C to 200 C, and preferably at a temperature within the range of 1 00 C to 150 C.

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6. Advantageously, and without the use of additional components such as epoxy resins, the prepreg compositions of the invention possess little or no tack. The exclusion of such components enables the prepreg compositions of the invention to be cured at the conveniently low temperatures referred to above.

5 The prepreg composition of the invention has been shown to exhibit good mechanical properties. Advantageously, the prepreg composition of the invention may achieve these properties without the need for any additional cross linking agents.

The prepreg compositions of the invention are converted into the partially l O cured form by a B-stage reaction in which at least a proportion of the epoxy functional groups of the resin monomer molecules undergo a polymerization reaction with the epoxy curing agent present in the composition. This reaction preferably occurs in the range 0 C to 40 C and most preferably occurs at room temperature (20 C to 25 C).

The degree of tack possessed by a partially cured form of the prepreg 15 composition can be varied by controlling the amount of the epoxy curing agent available for the B-stage reaction. It is preferable, therefore, that there is a sufficient quantity of the epoxy curing agent present in the composition to produce a partially cured prepreg composition having the desired property of possessing little or no tack.

Suitable examples of epoxy curing agents that could be used in the prepreg 20 compositions of the invention include isophoronediarnine (IPOA) ; 1,2

À 4 t À ' À À

cyclohexanediamine; p-menthanediamine; N-aminoethylpiperazine; 4,4' methylenebis(cyclohexylamine); 4,4'-methylenebis(2-methylcyclohexylamine); and 2,4,6-tris(dimethylaminomethyl)phenol. However, any suitable epoxy curing agent (or B-staging agent), known for the cure of epoxy resins at close to ambient 5 temperatures, could be used.

As previously mentioned, the partially cured prepreg can be readily converted into a fully cured fibre-reinforced composite by heating the composition. Heating the partially cured composition to an elevated temperature causes the resin to "filly cure" by the initiation of a second cure reaction which involves the polymerization andJor 10 cross- linking of the acrylate functional groups present in the resin monomer molecules. This reaction is catalysed by the vinyl ester curing agent present in the composition. Vinyl ester curing agents are free-radical initiators which catalyse the polymerization of the functional acrylate groups of the resin monomer molecules. An 15 example of a suitable free radical-generating curing agent that could be used in the compositions of the invention is 2,2'-azobisisobutyronitrile (AIBN), although any other suitable vinyl ester curing agent could be used in the compositions of the present invention. For example, azobis(cyclohexanecarbonitrile), azobis(4-methoxy-2,4-

dimethylvaleronitrile), azobis(2,4-dimethylvaleronitrile), dimethyl-2,2'azobis(2 20 methylpropionate), benzoylperoxide, lauroylperoxide and the like are other suitable free radical initiators.

I Al Wl- ttItI; lt41 8 ' There may also be present in the assembly an inhibitor for the free radical polymerization reaction to enable longer storage life to be achieved after B-staging.

Such compounds may include hydroquinone, methoxyhydroquinone, butylated hydroxytoluene, phenothiazine and other inhibitors well known in the art.

5 It may also be desirable to add promoters or accelerators to the assembly in order to increase the rate of the final cure reaction. Such accelerators may include cobalt and nickel salts including cobalt naphthenate or cobalt(II) 2-ethylhexanoate.

Promoters may include amines such as N,N-dimethylaniline or N,N diethylacetoacetamide. 10 Preferably, the resin monomer and the epoxy and vinyl ester curing agents are thoroughly mixed to provide a substantially uniform distribution of each constituent throughout the resin composition.

In preferred resin monomers of the invention, L is a linking group having a structure represented by the general structural formula TI below: R3 R: R8 R5

-o Cot wherein

À I: À. À a: l À À c: À R3, R4, R5 and R6 are each independently selected Dom H. Cl, Br or Ci-C5 alkyl; and R7 and R' are each independently selected from H. Cl, Br or C'-C5 alkyl; or R7 and Rg may together form part of a ring structure, for example a cyclohexyl ring.

5 Especially preferred resin monomers are those in which L has the structural formula II shown above and R' is either H or methyl, R2 is H.; R3, R4, R5 and R6 are H.; and R7 and Rg are methyl. When R' is H. the compound is known as monoepoxyacrylate (MEA) and has the structure represented by the structural formula III below.

' o III O OH When Ri is methyl, the compound is known as monoepoxyrnethacrylate (MEMA) and; has the structure represented by structural formula IV below.

ooo IV 15 o

:. t.. À.;:.:e:c Suitable resin monomers of the invention can be readily synthesised by the reaction of acrylic acid, or a substituted derivative thereof, with a substantially bifunctional epoxy resin such as a bisphenol A diglycidyl ether, a bisphenol F diglycidyl ether, a resorcinol diglycidyl ether, epoxycyclohexylmethyl 5 epoxycyclohexanecarboxylate, or a substituted derivative thereof. Alternatively, a multifunctional epoxy resin such as a novolak epoxy or the like could also be used.

The fibrous reinforcement present in the prepreg compositions may comprise organic or inorganic fibres such as, for example, carbon, glass, ararnid, polyethylene, polypropylene, ceramic, or natural fibres such as kenaf, hemp or cellulose, or any 10 combinations thereof. The fibres may be utilised in unidirectional form, or as non-

wovens such as multi-axial fabrics or non-crimped fabrics, or may be present as a woven fabric or non-woven mat or fabric, or combinations thereof.

Certain additional constituents may also be present in the compositions of the present invention such as, for example, one or more fillers, accelerators, 15 thermoplastics and core shell rubbers, flame retardants, pigments/dyes, W absorbers, toughening particles etc. In certain preferred compositions of the invention, however, the resin matrix is a single component resin matrix, i.e. the resin matrix comprises the resin monomer as the sole polymeric resin matrix-forming constituent thereof.

. . À.

À. : c:.: À ee e The prepreg compositions of the invention may additionally include an electrically conductive mesh for dissipating electrical energy from lightening strikes.

The prepreg compositions of the invention may also be coated on at least one surface with a surface -finishing film or coating which provides the required aesthetic 5 finish to the composition and/or provides a protective barrier between the prepreg composition and the surrounding environment.

According to a second aspect of the present invention there is provided a process for the preparation of a prepreg composition, said process comprising the steps of: 10 (i) mixing a resin monomer of formula I as hereinbefore defined together with an epoxy curing agent and a vinyl ester curing agent to form a resin matrix; and (ii) impregnating a fibrous reinforcement with said resin matrix.

According to a third aspect of the present invention there is provided a process 15 for the preparation of a partially cured form of a prepreg composition, said process comprising the steps of: (i) mixing a resin monomer as hereinbefore defined together with an epoxy curing agent and a vinyl ester curing agent to form a resin matrix;! 20 (ii) impregnating a fibrous reinforcement with said resin matrix; and

.' l. À -.: e Àe.:e::: 12 1 (iii) allowing said resin matrix to partially cure by the reaction of said resin monomer with said epoxy curing agent.

According to a fourth aspect of the present invention there is provided a process of preparing a fully cured fibre-reinforced composite from a prepreg or 5 partially cured composition as hereinbefore defined, said process comprising heating said prepreg or partially cured composition to a temperature within the range of 60 C to 200 C. This preferably occurs over a time period of between 1 minute and 16 hours. t Ideally the aforesaid resin monomer of formula I is substantially the sole polymeric resin matrix-forming constituent of said composition; i.e. it constitutes at 10 least 90% by weight of the polymeric resin matrix-forming constituent of said composition. It will be understood that by "sole polymeric resin matrix-forming constituent" we mean that composition comprises (i) substantially no other resin monomers other than those of the present invention as hereinbefore defined, and/or (ii) no polymeric 15 material capable of reacting with, or coupling to, the resin monomers of the invention to form a constituent of the polymeric resin matrix. This does not preclude the presence of other "non-polymeric resin matrixforming constituents" been either mixed with, or entrapped within, the resin matrix.

. À. À À:

. À À À À Preferably, the resin composition consists essentially of a resin monomer as hereinbefore defined together with an epoxy curing agent and a vinyl ester curing agent. The preparation of a resin composition according to the present invention will 5 now described in more detail in reference to the specific example set out below.

Example t

1. Preparation of monoepoxymethacrylate (MEMA) MEMA was prepared by heating at 80 C equimolar amounts of a commercially available bisphenol A epoxy and methacrylic acid. A typical 10 preparation is as follows; to DER332, a bisphenol A epoxy resin available from Dow (136.00g, 0.4 mol), were added triphenylphosphine (0.80 g, 3.1 mmol) and 4-methoxyphenol (0. 40 g, 3.2 rnmol). This mixture was heated at 80 C for 30: minutes. Methacrylic acid (34.40 g, 0.4 mol) was added and the mixture was stirred and heated at 80 C for 6 hours. On cooling to room temperature a viscous orange 15 resin was isolated. The as synthesised MEMA can then be used without further purification. 2. Preparation of a tack free resin composition suitable for use in the preparation of a tack free prepreg composition

A; e The following constituents were uniformly mixed at room temperature and applied to glass fabric (weight 700 gm2). The matrix was then allowed to B-stage at room temperature for 12 hours to give a tack free prepreg.

MEMA 20.0g 5 Isophoronediamine (IPDA) 2.00g 2,2'-Azobisisobutyronitrile (AIBN) 0.40g The isophoronediarnine (IPDA) reacts with the epoxy groups of the MEMA molecules at room temperature to provide a substantially tack free composition. Less IPDA can be added to produce a composition possessing a degree of tack.

10 The IPDA could be substituted with any another suitable B-staging agent/epoxy curing agent such as, for example, 2,4,6-tris(dimethylarnino methyl)phenol. The resin composition is then fully cured by heating the composition at a temperature of 100 C for up to 15 minutes. At this temperature, the AIBN acts as the initiator for 15 the polymerization of mcthacrylate groups of the MEMA molecules to yield the final cured resin matrix. The Tg of this cured resin was found to be 80 C as determined by dynamic mechanical thermal analysis.

It is to be understood that the above described example is by way of I illustration only. Many modifications and variations are possible.

Claims

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Àe.e a À À À À

À. À

Claims

1. A resin composition comprising a fibrous reinforcement impregnated with a resin matrix, said resin matrix further comprising: ] (i) a resin monomer having the general structure formula I shown below: R2 (CHR9-L-)J I

5 R in which: L is a linking group which may comprise aliphatic, aromatic or alicyclic units; R' is H. CN or C-C5 alkyl; R2 is H or together with R9 forms part of a saturated ring of five or six carbon l O atoms; R9 is H or together with R2 forms part of a saturated ring of five or six carbon atoms; (ii) a curing agent ror the epoxy group; and

el. te.. À.e I::..:e (iii) a curing agent for the vinyl ester group.

2. A resin composition according to claim 1, wherein the curing agent for the epoxy group is selected from any of the following either alone or in combination:- isophoronediamine (IPDA); 1,2-cyclohexanediamine; p-

5 menthanediarnine; N-aminoethylpiperazine; 4,4'methylenebis(cyclohexylamine); 4,4'-methylenebis(2-methylcyclohexylarnine) ;and 2,4,6-tris(dimethylarninomethyl) phenol. 3. A resin composition according to claim 1 or claim 2, wherein the curing agent for the vinyl ester group comprises at least one free-radical initiator.

10 4. A resin composition according to claim 3, wherein the free-radical initiator is selected from any of the following either alone or in combination:- 2,2'-

azobisisobutyronitrile (AIBN), azobis(cyclohexanecarbonitrile), azobis(4methoxy-

2,4-dimethylvaleronitrile), azobis(2,4-dimethylvaleronitrile), dimethyl-2, 2'-azobis(2-

methylpropionate), benzoylperoxide and lauroylperoxide.

15 5. A resin composition according to any preceding claim, wherein the composition further comprises a free-radical polymerization inhibitor.

6. A resin composition according to claim 5, wherein the inhibitor is selected from any of the following either alone or in combination:hydroquinone, methoxyhydroquinone, butylated hydroxy toluene and phenothiazine.

l e À - À :.e I:::: À: À 7. A resin composition according to any preceding claim, wherein the composition further comprises at least one cure promoter or cure accelerator.

8. A resin composition according to claim 7, wherein the cure accelerator comprises at least one cobalt salts or nickel salt.

5 9. A resin composition as claimed in claim 7 wherein the cure accelerator comprises at least one of cobalt naphthenate and cobalt (II) 2-ethylhexaneate.

10. A resin composition according to claim 7, wherein the cure promoter comprises at least one amine.

11. A resin composition as claimed inc laim 7, wherein the promoter comprises at 10 least one of N,N-dimethylaniline or N,Ndiethylacetoacetamide.

12. A resin composition according to any preceding claim, wherein the linking group of the resin monomer is of general formula II below: R3 R( R8 R5

-00 I}

R4 R6 OH wherein: 15 R3, R4, R5 and R6 are each independently selected from H. Cl, Br or Ct-C5 alkyl; and

; e R7 and R8 are each independently selected from H. C1, Br or Cl-C5 alkyl; or R7 and R8 may together form part of a ring structure.

13. A resin composition according to claim 12, wherein the linking group is of gerieral formula II, in which: 5 R' is either H or methyl; R2 is H.; R3 = R4= Rs = R6 = H; and R' = R8 = methyl.

14. A resin composition according to any preceding claim, wherein the 10 composition comprises one or more additional constituents selected from; fillers, accelerators, thermoplastics and core shell rubbers, flame retardants, pigments/dyes, UV absorbers, toughening particles.

15. A process for the preparation of a prepreg composition, said process comprising the steps of: 15 i) mixing a resin monomer of formula I (shown below) together with an epoxy curing agent and a vinyl ester curing agent to form a resin matrix; and ii) impregnating a fibrous reinforcement with said resin matrix.

À À. ...

À À À R2 CHR9 L-O t R in which: L is a linking group which may comprise aliphatic, aromatic or alicyclic units; R' is H. CN or C-C5 allcyl; 5 R2 is H or together with R9 forms part of a saturated ring of five or six carbon atoms; and R9 is H or together with R2 fonns part of a saturated ring of five or six carbon atoms. 16. A process for the preparation of a partially cured form of a prepreg 10 composition, said process comprising the steps of: i) mixing a resin monomer (shown below) together with an epoxy curing agent and a vinyl ester curing agent to form a resin matrix; ii) impregnating a fibrous reinforcement with said resin matrix; and

:e at' me;.:e:e iii) allowing said resin matrix to partially cure by the reaction of said resin monomer with said epoxy curing agent.

R2 Icy- I in which: 5 L is a linking group which may comprise aliphatic, aromatic or alicyclic units; R' is H. CN or Cat - Cs alkyl; R2 is H or together with R9 forms part of a saturated ring of five or six carbon atoms; and R9 is H or together with R2 forms part of a saturated ring of five or six carbon 1 0 atoms.

17. A process according to claim 16, wherein the composition is partially cured in the temperature range from 0 C to 40 C.

18. A process according to claim 17, wherein the composition is partially cured in the temperature range from 20 C to 25 C.

_ À À r 19. A process of preparing a fully cured fibre-reinforced composite from a prepreg or partially cured resin matrix composition as hereinbefore defined, said process comprising heating said prepreg or partially cured resin matrix composition to a temperature within the range of 60 C to 200 C.

5 20. A process according to claim 19, wherein the prepreg or partially cured composition is heated to a temperature within the range of 100 C to 150 C.

21. A process according to claim 19 or claim 20, wherein the heating process occurs over a time period of between 1 minute and 16 hours.