SILOXANE-IMIDE BLOCK COPOLYMERS FOR TOUGHENING EPOXY RESINS

BACKGROUND OF THE INVENTION

Field of the Invention

Epoxy resins are commonly used in a wide range of products and end-uses. These include products such as composite resins, adhesives and coatings in applications such as aerospace, construction, electronics and general engineering. A drawback of many epoxy resins, particularly those with high use temperatures, is their inherent lack of toughness due to their crosslinked thermoset nature.

A lot of studies have focused on ways of improving the toughness of epoxy resins while maintaining other desirable properties. Invariably, some trade-off of properties is required and accepted in these cases. Also, for applications such as aerospace, a large proportion of toughening additive is often needed, resulting in a significantly increased cost. Our invention is aimed at producing a tough epoxy resin with relatively low addition level of toughening agent.

Description of the Prior Art

The two most common ways of toughening epoxy resins are addition of a thermoplastic and addition of a rubber toughening agent. Both have been fully described in the patent and open (scientific) literature. For example, Hitco patent application WO 91/02029 describes the use of polyimides to toughen epoxy resins. Bucknall and Gilbert (C B Bucknall £_ A H Gilbert, Polymer, 1989, _i0 _/ 213) describe the use of polyetherimide to toughen tetrafunctional epoxy resin. Rubber toughening of epoxy resins is described in inter alia R Pucciariello, V Villani, N Bianchi and R Braglia, Die Angew. Makromol. Chemie, 1991, 187, 75. Both thermoplastic and rubber methods find use in commercial epoxy resin systems.

Very large increases in toughness are possible using rubbers such as carboxy-terminated butadiene-acrylonitrile copolymer (CTBN) , but modulus is reduced and thermal stability of the product compromised. Use of inorganic siloxane polymers and copolymers as rubber- oughening agents can circumvent the problem of reduced thermal stability and also lead to improved moisture resistance.

In European Patent Application EP 475611, Dow Corning and Toray describe the use of amino-functional silicones to toughen epoxy resins. Similarly, amino- erminated siloxanes have been used to produce low stress epoxies used in electronic applications (T Takahashi et al, Rubber-Toughened Plastics, Adv. Chem. Ser. 222, ACS, 1989, p. 243) . Wacker Chemie have also described (European

Patent Application 415204) the use of anhydride-terminated siloxane polymers as hardeners for epoxy resins. Compatibility with epoxy resin of dimethylsiloxane oligomers is poor and is improved by incorporation of phenylsiloxane units in the elastomer chain.

Toray has described (European Patent Application EP 382 575) the use of siloxane-imide copolymers to toughen epoxy resins. In this patent application, Toray claim the use of block and graft copolymers (only statistical block copolymers are illustrated) 'comprising a molecular chain compatible with, and a molecular chain incompatible with, the epoxy resin' .

A statistical block copolymer is one where a preformed block in the form of a 'm cromonomer' is reacted with other comonomers to form a type of random block copolymer. A preformed block copolymer is one wherein the oligomeric blocks are preformed and then reacted together to give a copolymer containing a regular sequence of well defined blocks.

The structure and size of the statistical block copolymer segments disclosed in Toray is determined by a combination of statistical and kinetic factors. The structure of the Toray copolymers is therefore ill-defined with the copolymers having a distribution of copolymer block sizes . The non uniformity of the copolymer blocks has an adverse effect on the toughening efficiency of the thermoplastic toughening agent. As a result a relatively large amount of the thermoplastic toughening agent is required thereby increasing the cost of the toughened resin.

In addition, the disclosure of Toray stresses throughout the need for three dimensionally continuous structures in both phases . Toray is concerned with a cured resin product including a thermoplastic resin and a thermoset resin with the thermoplastic and thermoset forming separate phases, each phase having a three dimensionally continous structure. The resulting structure of the cured resin may be considered to be similar to a wet sponge wherein both the sponge and water are three dimensionally continuous and represent the separate phases. Such a structure will only result when there is sufficient of the thermoplastic toughening agent to form a three dimensionally continous structure spreading throughout the thermoset's three dimensionally continuous structure. A threshold concentration of thermoplastic toughening agent must therefore be reached for the co-continous structure of Toray to result. This threshold concentration may be described as the co-continuity limit. Below the co-continuity limit the toughening agent will not form a three dimensionally continuous structure. The necessary concentration of thermoplastic toughening agent in Toray (co- continuity limit) lies within the range 10-50% by weight of the total weight of the resin composition.

Any possible reduction in the amount of toughening agent required will reduce the cost of the resin composition and therefore represent a clear improvement over the prior art.

The presence of a bulky organic moiety such as the fluorene group also appears to be preferred in Toray.

Wacker Chemie (European Patent Application EP 415204) describe the synthesis of similar siloxane-imide statistical block copolymers, but not their use in toughening other polymers. In European Patent Application EP 273150, General Electric describe the synthesis of siloxane-imide block copolymers and their use as impact modifiers in engineering thermoplastics, but not thermosets.

It is accepted by those skilled in the art that the toughening of epoxy resins by thermoplastics is governed by empirical rules rather than precise scientific rules. Small changes in chemical structure can sometimes (unexpectedly) produce dramatic changes in toughening efficiency. It therefore follows that it is not possible to predict with certainty that a given thermoplastic will produce a specific toughening effect .

Our invention was aimed at producing a resin system showing high toughness at low toughening agent loading levels (for cost reasons) for use in commercial composite applications such as transport, construction and general engineering. The base resin was a cheap, commercially available epoxy resin system, but any epoxy resin could be used. We targeted preformed siloxane- imide block copolymers, such as AB and ABA copolymer^, as toughening agents. Because the blocks would be preformed, this would allow better control of polymer structure and block size and also allow the polymer structure to be tailored for toughening efficiency. Higher than expected values of fracture toughness have been obtained for these resin systems.

SUMMARY OF THE INVENTION

The present invention provides a. resin formulation including a thermoset resin and a thermoplastic resin wherein the thermoplastic resin is a preformed siloxane-imide block copolymer.

The object of the invention is to devise an efficient way of toughening epoxy resins for use in commercial composite applications . The aim is to achieve high fracture toughness without sacrificing thermal stability or increasing water absorption.

The invention is concerned with epoxy resin compositions containing toughening agents which are preformed siloxane-imide block copolymers of the AB and ABA type and graft copolymers of the type A(g-B) _m . The block size of the copolymers is controlled by the synthetic route employed. It is important that the blocks are pre-formed to allow this control of structure. [In statistical block copolymers, the structure of the segments which are not pre-formed is controlled by a combination of statistical and kinetic factors, which leads to structures which are ill- defined and not necessarily uniform.] The use of toughening agents with uniform block sizes is believed to lead to improved

toughening efficiency. As little as 10% w/w of toughening agent is required to achieve significant increases in fracture toughness. No optimum phase morphology is required. Indeed, non-co-continuous morphology is preferred in these systems. It is surprising that block compatibility with the epoxy is not important and that copolymers with both epoxy compatible and/or incompatible blocks can be used. It is also found that the mechanical properties of the toughened resin can be controlled via adjustment of the prereaction processing conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a scanning electron micrograph of a resin sheet produced as described in example 1, part III;

Figure 2 shows a scanning electron micrograph of a resin sheet produced as described in example 3, part III; and

Figure 3 shows a scanning electron micrograph of a resin sheet produced as described in example 6, part II.

DESCRIPTION OF THE INVENTION

In resins embodying the present invention an epoxy matrix formulation contains and is toughened by preformed siloxane-imide block copolymers of the AB and ABA type and graft copolymers of the type A(g-B) _m .

The toughening agent is chosen from block copolymers of structures ABA and AB and graft copolymers of structure A(g-B) _m where m=l-20 and both A and B are oligomeric blocks which are preformed prior to coupling. The chemical identities of A and B are such that one of either A or B is a siloxane (silicone) polymer as described below(i) and the other is an imide polymer as described below(ii) . The molecular weights of blocks A and B lie in the range 1000-1000000.

(i) Siloxane oligomer. Comprising a material of structure X- t-Si(R _x ) (R ₂ ) -O-] -Si (R.) (R ₂ ) -Y

where R _x may or may not have the same identity as R ₂ or X or Y and R _x , R ₂ are composed of alkyl or aryl constituents and X may or may not have the same identity as Y and X, Y comprise structure R ₃ -Z where R ₃ may or may not have the same identity as R. or R ₂ and R ₃ is composed of alkyl or aryl constituents

and Z is a reactive (e.g. amine, acid, acid anhydride, hydroxyl, thiol, isocyanate) or unreactive (e.g. aryl, alkyl) moiety

(ii) Imide oligomer. Comprising a material of structure

where D is a dianhydride residue selected from those shown in Table Tl and E is a diamine or diisocyanate residue selected from those shown in Table T2 and A may or may not have the same identity as B where A and/or B are reactive (e.g. amine, acid or acid anhydride) or unreactive (e.g. aryl, alkyl) terminal units

Excepting when units A, B and Z are all unreactive.

Table Tl- Structural variants ^' for the dianhydride residue.

Table T2. Structural variants for the diamine/diisocyanate residue.

^<y

ΕX

where R'= H or CH ₃ or C ₂ H ₅ ,

The epoxy matrix formulation consists of upto 6 parts (detailed as parts A to F below) and is cured by thermal or radiation (including u.v. and microwave) methods to the final product.

Part A. An epoxy or epoxy-novolac resin, or a combination thereof, available from commercial sources.

Part B. A curing agent suitable for the resin(s) selected.

Part C. The copolymer toughening agent as described earlier in a proportion from

1% by weight of parts A and B upto the co-continuity limit.

Part D. Optionally a cure accelerator or promoter.

Part E. Optionally a matrix modifier (e.g. a reactive or non-reactive diluent or plasticiser or a HDT improver or a combination thereof) .

Part F. Optionally a processing aid (e.g. a defoaming agent or a low shrink additive or a release agent or a stabiliser to protect against thermal and environmental influences) .

The epoxy matrix formulation of the invention may be used in the production of a composite material. Such a composite may be fibre reinforced, filler reinforced or reinforced by a combination of fibre and filler. Suitable reinforcing fibres include glass, carbon, kevlar, nylons, aramids, thermoplastic fibres or inorganic fibres. Suitable reinforcing fillers include calcium carbonate, talc, clay, silica and alumina trihydrate.

The main advantages of the invention are:-

1) Applicability to a range of preformed block and graft copolymer structures, irrespective of block compatibility with the epoxy resin.

2) The phase morphology need not be controlled to give co- continuous structures; indeed, the production of systems with non-co-continuous structures is a feature of the invention.

3) Ability to control copolymer block size and structure through the use of preformed blocks leads to improved toughening efficiency.

4) A high level of fracture toughness is obtained at low levels of toughening polymer loading.

5) Improved thermal stability and reduced water absorption compared to conventional rubber toughened systems.

6) Vibration damping capability due to presence of an elastomeric component .

DESCRIPTION OF PREFERRED EMBODIMENTS

EXAMPLES

The invention is further illustrated by the following examples which are not intended to be limiting.

Example 1.

Part I

11.51g (0.029 mol) of 4,4' -(1,4 - phenylenebis (2-propyl) ) -bis (2,6 dimethyl aniline) (Bis-XLDP) was dissolved in 147g of N- methyl-2-pyrolidone (NMP) and 11.15g (0.03 mol) of diphenylsulphone tetracarboxylic dianhydride (DSDA) was added. The mixture was stirred at room temperature under a nitrogen atmosphere for 16 hours. 22g of xylene were added to the flask and the temperature raised until azeotropic distillation occurred. Distillation was continued for 6 hours and the mixture allowed to cool at room temperature under a nitrogen atmosphere. The reaction mixture was poured slowly into 1.51 of methanol to precipitate the anhydride-terminated polyimide oligomer as a solid product. This solid was stirred in 1.51 of water, filtered and washed with copious quantities of methanol. The solid was dried in stages at increasing temperatures upto 200°C. ..Yield 19.37g (89%) of beige-coloured powder.

Part II

10.77 g (0.0012 mol) of polyimide oligomer from part I was dissolved in 60g of NMP and added to 13.23g (0.0024 mol) of an amino-terminated dimethyl siloxane (commercially available from Wacker Chemie, SLM 55019/1) dissolved in a mixture of 60g of NMP and 30g of N-cyclohexyl-2-pyrolidone (CHP) . The reaction mixture was stirred at 105°C under a nitrogen atmosphere for 16 hours. The temperature was then raised and azeotropic dehydration carried out for 6 hours. The cooled reaction mixture was precipitated and the precipitate extracted, as in part I. Drying was carried out to a temperature of 150°C. Yield 21.78g (91%) of fawn-coloured powder.

Part III

50.00g of bisphenol-A diglycidyl ether (commercially available from Ciba-Geigy, MY750) was stirred at 40 C and 7.50g of siloxane-imide-siloxane copolymer from part II was added in dichloromethane . The temperature was raised in stages to 120°C with thorough vacuum degassing and lβ.OOg of dia inodiphenylsulphone (DDS) added in portions. The mixture was stirred at 120°C for 30 mins before being transferred to steel moulds which had been pretreated with release agent and heated to 120°C. The curing reaction was carried out in an oven at 130°C for 3 hours and 200°C for 2 hours to produce a cured resin sheet of thickness greater than 4mm. This sheet was milled to 4mm thickness and cut to the appropriate size test pieces. The

fracture surface energy G _IC (EGF protocol March 1990 - based on ASTM E-399) was measured to be 0.64 kJ/m ² and the flexural modulus (ASTM D790M) was 2.70 GPa. The compression yield strength (ISO 604) was found to be 89.5 MPa. The Tg of the cured resin was 206 ^α C.

Part IV

The procedure and materials detailed in part III were repeated except in one respect - after mixing of the MY750 and copolymer from part II at 40°C, the temperature was quickly raised to 120°C. The remainder of the procedure remained as detailed in part III. The measured properties were G _IC 0.38 kJ/m ² , flexural modulus 2.58 GPa, compression yield strength 108 MPa and Tg 208«C.

Example 2.

Part I

16.48g (0.046 mol) of DSDA and 17.63g (0.044 mols) of Bis-XLDP were the monomers used. The procedure was as in example 1, part I.

Part II

3.36g (0.002 mol) of amino-terminated dimethylsiloxane (commercially available for Shin-Etsu Chemical, X22-161A) and 16.69g (0.001 mol) of imide oligomer from part A were the monomers used with NMP and xylene as the solvents . The procedure to form the siloxane-imide-siloxane copolymer was as in example 1, part I. Yield 16.07g (80%) of a pinkish-coloured powder.

Part III

Blending and machining as in example 1, part III. G _IC measured to be 0.24 kJ/m ² , flexural modulus 2.76 GPa and compressive yield strength 114.6 MPa. Resin blend Tg was 220°C.

Example 3.

Part I

15.16g (0.044 mol) of 4 , 4' (1, 3-phenylene bis (2-propyl) bisaniline)

(BisM) and 13.53g (0.046 mol) of biphenyltetracarboxylic dianhydride (BPDA) were used as described in example 1, part I.

Part II

4.03g (0.0024 mol) of X22-161A and 20.06g (0.0012 mol) of imide oligomer from part I were used as raw material as in example 2, part II. Yield 20.48g (86%) of a yellow powder.

Part C.

Blending method and machinery of resin sheet were as in example

1, part III. Measured properties were G _IC 0.26kJ/m ² , flexural modulus 2.96 GPA, co pressive yield strength 112.9 MPa, Tg 218°C.

Example 4.

Part I

7.50g (0.0045 mol) X22-161A and 0.66g (0.0045 mol) of phthalic anhydride (PA) were reacted together as in example 1, part I but the reaction mixture was not precipitated. Instead it was maintained at a temperature of 120°C under a nitrogen atmosphere. To this solution was added 16.16g (0.040. mol) BisXLDP and the mixture stirred until all solids had dissolved. I4.45g (0.040 mol) DSDA and 0.50g (0.0034 mol) PA were added and stirred until dissolved. This mixture was dehydrated azeotropically for 6 hours and allowed to cool to room temperature. This reaction product was then precipitated and isolated as in example 1. Yield 24.70g (68%) pale pink powder.

Part II

The blending and machining of produced resin sheets was carried out as in example 1, part III. Resin properties were G _IC 0.13 kJ/m ² , flexural modulus 2.84GPa, compressive yield strength 123.4MPa, Tg 22l°C.

Example 5.

Part I

The procedure of example 4, part I was followed using 7.50g (0.0045 mol) X22-161A, 0.66g (0.0045 mol) and 0.65g (0.0044 mol) PA, 22.80g (0.068 mol) BisM and 1.9.47g (0.066 mol) BPDA as the raw materials. Yield 35.59g (77%) yellow powder.

Part II

The blending, curing and testing schedule was carried out as described in example 1, part III. Measured properties were G _IC 0.21 kJ/m ² , flexural modulus 2.85GPa, compressive yield strength 75.1 MPa, Tg 223°C.

Example 6.

Part I

In separate reaction flasks; 3.36g (0.002 mol) X22-161A and 1.18g (0.004 mol) BPDA were reacted as described in example 1 part I but the reaction mixture was not precipitated; and 17.23g (0.050 mol) BisM and 13.53g (0.046 mol) BPDA were reacted as described in example 1 part I but the reaction mixture was not precipitated. The contents of the two flasks were combined and azeotropically dehydrated and isolated as described in example 1. Yield 30.38g (94%) of beige-coloured powder.

Part I I

The blending, curing and testing schedule adopted was as described in example 1, part III. Measured values were G _IC 0.23kJ/m ² , flexural modulus 2.891 GPa, compressive yield strength 115.0 MPa, Tg 217°C.

Example 7.

Part I

The procedure described in example 6, part I was followed using 3.36g (0.002 mol) X22-161A, 1.43g (0.004 mol) DSDA; and 20.03g (0.050 mol) Bis-XLDP and 16.48g (0.046 mol) DSDA as the raw materials. Yield 34.68g (93%) pinkish-coloured powder.

Part II

The blending and testing procedure in example 1, part III was followed. Measured properties were G _IC 0.17 kJ/m ² , flexural modulus 3.00 GPa, compressive yield strength 103.6 MPa, Tg 216°C.