Toughened adhesive and bonding method using the same

Abstract

A bonding method for joining two structural parts using a curable adhesive layer having a fibrous veil embedded therein. The fibrous veil carries a polymeric binder, which is in a solid phase at room temperature (20° C.-25° C.) and is capable of dissolving into the adhesive composition during curing thereof.

Claims

1. A bonding method comprising: (a) joining a first substrate to a second substrate with a curable adhesive layer between the substrates; and (b) curing the adhesive layer at a temperature above 25° C. to form a bonded structure, wherein the adhesive layer comprises a veil embedded in a curable matrix resin, the curable matrix resin comprises one or more thermoset resins and at least one curing agent, the veil comprises veil fibers and a polymeric binder, which is in a solid phase at room temperature (20° C.-25° C.); wherein the polymeric binder is distributed on the outer surfaces of the veil fibers and/or dispersed among the veil fibers throughout the veil, and during curing, the polymeric binder dissolves into the matrix resin.

2. The bonding method according to claim 1, wherein the polymeric binder is in the form of particles dispersed among the veil fibers throughout the veil.

3. The bonding method according to claim 1, wherein the polymeric binder is formed of a polymeric material selected from thermoplastic polymers, elastomeric polymers, and combinations thereof.

4. The bonding method according to claim 1, wherein the polymeric binder is formed of a thermoplastic polymer or copolymer having a glass transition temperature of less than 350° F., as determined by Dynamic Mechanical Analysis.

5. The bonding method according to claim 1, wherein the polymeric binder comprises a thermoplastic material selected from: polyamide, polyester, polyimide, polycarbonate, polyurethane, polyetherimide, polyethersulfone, poly(methyl methacrylate), polystyrene, polyarylsuphone, copolymers thereof, and combinations thereof.

6. The bonding method according to claim 1, wherein the polymeric binder comprises a copolymer of polyamide 11 (“PA11”) and polyamide 12 (“PA12”).

7. The bonding method according to claim 6, wherein the ratio of PA11 to PA12 is 20:80 to 80:20.

8. The bonding method according to claim 1, wherein the fibrous veil is a nonwoven veil, a woven fabric or a knitted mat.

9. The bonding method according to claim 1, wherein the fibrous veil has an areal weight of about 2 to about 150 gsm.

10. The bonding method according to claim 1, wherein the fibrous veil comprises polymeric fibers or glass fibers.

11. The bonding method according to claim 1, wherein the curable matrix resin of the adhesive layer comprises one or more epoxy resins and at least one amine curing agent.

12. The bonding method according to claim 11, wherein the curable matrix resin further comprises a toughening material selected from: thermoplastic polymers, elastomeric polymers, polymeric particles, core-shell rubber (CSR) particles, and combinations thereof.

13. The bonding method according to claim 1, wherein the first substrate is selected from a metallic substrate, a composite substrate, and a honeycomb structure, and the second substrate is selected from a metallic substrate, a composite substrate, and a honeycomb structure.

14. The bonding method according to claim 13, wherein the composite substrate is a prepreg or a prepreg layup comprising a plurality of prepregs arranged in a stacking arrangement, wherein each prepreg comprises continuous, unidirectional fibers impregnated with a thermosetting resin composition.

15. An adhesive film comprising a veil embedded in a curable matrix resin, wherein the curable matrix resin comprises one or more epoxy resins and at least one curing agent, the veil comprises randomly arranged polymeric veil fibers or glass veil fibers and a copolymer of polyamide 11 (“PA11”) and polyamide 12 (“PA12”) as a polymeric binder, which is in a solid phase at room temperature (20° C.-25° C.) and has a glass transition temperature of less than 350° F., as determined by Dynamic Mechanical Analysis; wherein the polymeric binder is distributed on the outer surfaces of the veil fibers and/or dispersed among the veil fibers throughout the veil.

16. The adhesive film of claim 15, wherein the curable matrix resin comprises one or more polyepoxides, an amine curing agent, and a toughening material selected from: polyarylsulphones with or without reactive functional groups, core-shell rubber (CSR) particles, elastomeric polymers, and combinations thereof.

17. The adhesive film of claim 15, wherein the curable matrix resin comprises one or more polyepoxides, an amine curing agent, and a pre-react product formed by reacting bisphenol A with an epoxy resin in the presence of a catalyst, core-shell rubber (CSR) particles, and a polyarylsulphone.

18. A bonded structure comprising two substrates joined to each other and an adhesive film interposed between the substrates, wherein at least one of the substrates is a metallic substrate, the adhesive layer comprises a veil embedded in a curable matrix resin, which comprises one or more thermoset resins and at least one curing agent, the veil comprises intermingled, randomly arranged polymeric or glass veil fibers and a polymeric binder in an amount sufficient for holding the fibers together, wherein the polymeric binder is distributed on the outer surfaces of the veil fibers and/or dispersed among the veil fibers throughout the veil, is in a solid phase at room temperature (20° C.-25° C.) and has a glass transition temperature of less than 350° F. as determined by Dynamic Mechanical Analysis.

19. The bonded structure of claim 18, wherein the two substrates are metallic substrates.

20. The bonded structure of claim 18, wherein the polymeric binder comprises a copolymer of polyamide 11 (“PA11”) and polyamide 12 (“PA12”).

Description

EXAMPLES

(1) The following Examples are provided to illustrate certain aspects of the present disclosure. In all cases, an epoxy-based film adhesive containing tougheners and curing agents, curable at 250° F. (121° C.), was used, and the support carrier was varied to assess the full effect of fabric on bonding performance.

Example 1

(2) An epoxy-based film adhesive was formed by preparing a resin formulation containing in parts by weight: 125 parts of diglycidyl ether of bis-phenol A containing pre-dispersed nano-sized, 25 parts of bisphenol A, 6.5 parts of polyethersulfone, 0.1 parts of triphenyl phosphine, 5 parts of an amine terminated PES-PEES copolymer, 4 parts, of cyanoguanidine, and 2 parts of methylene diphenyl bis(dimethyl urea).

(3) The resin formulation was coated onto a release paper at 140° F. to form an adhesive film with 0.06 psf (300 gsm) film weight. A polyamide (PA11) veil containing a polyamide binder was pressed into the adhesive film under vacuum until the veil is embedded. The amount of binder in the veil was less than 5% by weight. The polyamide binder had a T.sub.g of 60° C. (140° F.). Polyamide veils with different areal weights, 4 gsm, 10 gsm, and 20 gsm were used to form different adhesive films.

(4) For comparison, the polyamide veil with binder was replaced with a glass veil (areal weight of 6 gsm) to form a “control” adhesive film.

(5) Each adhesive film with the embedded veil was interposed between two aluminum panels, and the resulting laminate was cured at 3° F./min. to 250° F. for 90 min. at 40 psi.

(6) For bonding performance testing of the bonded panels, the following test methods were used: Wide area lap shear (WALS) test performed according to ASTM D3165; Metal-Metal (M-M) peel test performed according to ASTM D3167.

(7) Table 1 shows the test results for metal-metal bonding after Lap Shear Strength test and Peel Strength test.

(8) TABLE-US-00001 TABLE 1 4 gsm veil 10 gsm veil 20 gsm veil with with with 6 gsm glass polyamide polyamide polyamide Property (control) binder binder binder Lap Shear 3955 psi 4576 psi 4360 psi 4667 psi Strength on (27.27 MPa) (31.55 MPa) (30.06 MPa) (32.18 MPa) aluminum substrates @ 180° F. (82° C.) Lap Shear 3089 psi 3464 psi 3654 psi 3466 psi Strength on (21.30 MPa) (23.88 MPa) (25.19 MPa) (23.90 MPa) aluminum substrates @ 250° F. (121° C.) Hot/wet Lap 3161 psi 3770 psi 3417 psi 3320 psi Shear (21.8 MPa) (26.00 MPa) (23.56 MPa) (22.89 MPa) Strength on aluminum substrates @ 180° F. (82° C.) M-M Peel on 76 lb/in 77 lb/in 76 lb/in 76 lb/in aluminum (338 N/25 mm) (343 N/25 mm) (338 N/25 mm) (338 N/25 mm) substrates @ 75° F. (24° C.) M-M Peel on 52 lb/in 57 lb/in 63 lb/in 68 lb/in aluminum (231 N/25 mm) (254 N/25 mm) (280 N/25 mm) (303 N/25 mm) substrates @ 225° F. (107° C.)

(9) As shown in Table 1, the veil fabrics containing polyamide binder yielded significantly improved high temperature bond strength properties versus the control veil. The gain in bond strength was not accompanied by a decrease in peel strength as usually observed experimentally. Hot/wet performance was also improved using the veil containing the polyamide binder versus control, demonstrating the effectiveness of the binder-containing veil.

Example 2

(10) An adhesive film with a binder-containing polyamide veil was prepared as disclosed in Example 1. CYCOM 5320-1 tape prepreg was pre-cured with a polyester peel ply by heating at 3° F./min to 350° F. for 2 h at 80 psi. The cured prepreg was cut into two panels to be bonded. The peel ply was removed to provide a roughened, bondable surface. The two prepreg panels were bonded together using the adhesive film and curing was carried out as described in Example 1. The resulting bonded laminate was labeled as “Laminate A”.

(11) For comparison, another bonded laminate was prepared in the same manner using an adhesive film containing the same resin composition but a different 20 gsm polyamide veil, which contained styrene acrylic binder. The resulting bonded laminate was labeled as “Comparison Laminate”.

(12) Table 2 shows the test results for the bonded laminates after Lap Shear Strength test.

(13) TABLE-US-00002 TABLE 2 Comparison Laminate Laminate A (veil with styrene (veil with Property acrylic binder) polyamide binder) Lap Shear 5792 psi 6462 psi Strength (39.93 MPa) (44.55 MPa) @ 75° F. (24° C.) Lap Shear 886 psi 1839 psi Strength (6.11 MPa) (12.68 MPa) @ 250° F. (121° C.) Lap Shear 2264 psi 3463 psi Strength (15.61 MPa) (23.88 MPa) @ 180° F. (82° C.) following exposure

(14) Data in Table 2 demonstrates the improvement in bond strength of the bonded composite laminate when the veil containing the polyamide binder was present as compared to the bonded laminate containing the veil without the same binder. In particular, high temperature Lap Shear strength was substantially increased for Laminate A as compared to the Comparison Laminate.