One-part structural epoxy resin adhesives containing elastomeric tougheners capped with phenols and hydroxy-terminated acrylates or hydroxy-terminated methacrylates

Abstract

Structural adhesives are prepared from an elastomeric toughener that contains urethane and/or urea groups, and have some terminal isocyanate groups that are capped with a phenol and other terminal isocyanate groups that are capped with a hydroxy-functional acrylate or a hydroxy-functional methacrylate. In certain embodiments, the presence of both types of capping on the toughener leads to higher impact peel strengths and a greater level of cohesive failure, than when the toughener is capped with a phenol an hydroxy-functional acrylate or hydroxy-functional methacrylate alone.

Claims

1. A structural adhesive comprising: A) at least one epoxy resin; B) an elastomeric toughener represented by structure (I) ##STR00004## wherein p is from 1.5 to 8; wherein a portion of the A groups comprises a residue after removal from a hydrogen from a hydroxyl group of a hydroxy-functional acrylate or methacrylate; another portion of the A groups comprises a residue of a phenol; Y is a residue of an isocyanate-terminated prepolymer after removal of the terminal isocyanate groups; and Y comprises at least one elastomeric segment; C) one or more epoxy curing agents; D) a liquid rubber-modified epoxy resin, a core-shell rubber, or both a liquid rubber modified epoxy resin and a core-shell rubber; E) a catalyst; and F) a latent free radical initiator that can be activated by radiation or by heating to a temperature of from 80 to 130 C.

2. A cured composition obtained by curing the structural adhesive of claim 1.

3. The cured composition of claim 2, having a Young's modulus of about 1000 MPa or greater.

4. The cured composition of claim 2, having a tensile strength of about 20 MPa or greater.

5. The structural adhesive of claim 1, which when cured has a Young's modulus of about 1000 MPa or greater.

6. The structural adhesive of claim 1, which when cured has a tensile strength of about 20 MPa or greater.

7. The structural adhesive of claim 1 which, when applied to cold rolled steel or galvaneal and cured to obtain a cured layer 1.5 mm thick, has a lap shear strength of about 15 MPa or greater.

Description

TOUGHENER EXAMPLES 1-9 AND COMPARATIVE TOUGHENERS A AND B

(1) A prepolymer is prepared by mixing, under nitrogen, 77.7 parts of a 2000 molecular weight polytetrahydrofuran, 0.5 part of trimethylolpropane and 0.2 part of a tin catalyst and heating at 85 C. until a homogeneous mixture is obtained. 13 parts hexamethylene diisocyanate are added and the mixture is allowed to react under nitrogen at 85 C. for 45 minutes. 0.01 part of an antioxidant is added, and the mixture is stirred for another five minutes at 85 C. The resulting prepolymer has an isocyanate content of 3.0%.

(2) The prepolymer is then mixed with 0.1 part of 2-hydroxyethylmethacrylate and 8.7 parts of o-allylphenol under nitrogen. The mixture is allowed to stir for 20 minutes to allow the capping reaction to complete. The resulting toughener (Example 1) is degassed under vacuum. 1.5% of the terminal isocyanate groups of Toughener Example 1 are methacrylate-capped, and 98.5% of the terminal isocyanate groups are o-allylphenol-capped.

(3) Toughener Examples 2-9 are made in the same general manner, except that the proportions of capping agents are changed in each case, as indicated in Table 1 below.

(4) Comparative Toughener A is made in the same general manner, except that all of the terminal isocyanate groups are capped with o-allyl phenol. Comparative Toughener B is made in the same general manner, except that all of the terminal isocyanate groups are capped with 2-hydroxylethylmethacrylate.

(5) Number average and weight average molecular weights for each of the tougheners are determined by gel permeation chromatography. The capping groups, molecular weights and polydispersity (M.sub.w/M.sub.n) for each of toughener Examples 1-9 and Comparative Tougheners A and B are reported in Table 1.

(6) TABLE-US-00001 TABLE 1 Ex. or % HEMA.sup.1 % o-allyl Comp. capping, phenol capping, Samp. No. mol-% mol-% M.sub.n.sup.2 M.sub.w.sup.2 PDI.sup.2 A* 0 100 5025 11,100 2.21 1 1.5 98.5 5430 11,610 2.14 2 5 95 5720 11,740 2.05 3 10 90 5375 11,660 2.17 4 20 80 5910 12,060 2.05 5 50 50 5715 11,590 2.03 6 80 20 6335 11,790 1.86 7 90 10 5610 10,930 1.95 8 95 5 5960 11,190 1.88 9 98.5 1.5 5950 11,170 1.88 B* 100 0 5835 10,610 1.82 *Not an example of this invention. .sup.1HEMA is 2-hydroxymethylmethacrylate. .sup.2M.sub.n is number average molecular weight of the toughener, M.sub.w is weight average molecular weight of the toughener; PDI is polydispersity, M.sub.w/M.sub.n. Molecular weights are by GPC.

(7) The data in Table 1 indicates that the various tougheners are all very similar in molecular weight. There is a trend towards higher polydispersity with greater o-allyl phenol capping levels.

ADHESIVE EXAMPLES A1-A9 AND COMPARATIVE ADHESIVES C-A AND C-B

(8) One-part, heat activated adhesive formulations are prepared from each of Toughener Examples 1-9 and Comparative Tougheners A and B, using the following formulation:

(9) TABLE-US-00002 Component Parts By Weight Diglycidyl ether of bisphenol A 54.5 Epoxy-terminated rubber.sup.1 13.8 Toughener 13.8 Dicyandiamide 4.3 Accelerator.sup.2 1.3 Fumed Silica 5.4 Fillers/Colorants 5.1 Versatic Acid monoepoxy ester.sup.3 1.3 Glycidyl silyl ether 0.8 .sup.1An adduct of a carboxyl-terminated butadiene-acrylonitrile rubber (HycarX13), bisphenol A based epoxy resin and cashew nut oil. .sup.2Tris (2,4,6-dimethylaminomethyl)phenol in a poly(vinylphenol) matrix. .sup.3Cardura E10, available from Christ Chemie.

(10) Adhesive Examples A1-A9 contain Toughener Examples 1-9, respectively. Comparative Adhesive C-A contains Toughener A and Comparative Adhesive C-B contains Toughener B.

(11) Impact peel testing is performed in accordance with ISO 11343 wedge impact method. Testing is performed at an operating speed of 2 m/sec. Impact peel testing is performed at 23 C., and strength in N/mm is measured.

(12) Test coupons for the impact peel testing are 90 mm20 mm with a bonded area of 3020 mm. The samples are prepared by wiping them with acetone. A 0.15 mm10 mm wide Teflon tape is applied to the coupons to define the bond area. The structural adhesive is then applied to the bond area of latter coupon and squeezed onto the first coupon to prepare each test specimen. The adhesive layer is 0.2 mm thick. Duplicate samples are cured for 30 minutes at 180 C.

(13) Duplicate test coupons are prepared and are evaluated for lap shear strength in accordance with DIN EN 1465. Testing is performed at a test speed of 10 mm/minute. Testing is performed at 23 C. Test samples are prepared using each adhesive. The bonded area in each case is 2510 mm. The adhesive layer is 0.2 mm thick. Duplicate test specimens are cured at for 30 minutes at 180 C.

(14) Results of lap shear strength an impact peel strength testing on oily cold rolled steel substrates (1.5 mm thick CRS 14O3 steel coated with Renoform MCO 3028 oil (Fuchs)) are indicated in Table 2. In this and the following examples, failure mode is described as BF (boundary failure), which is a cohesive failure close to the surface of the substrate or as CF (cohesive failure). Samples which exhibit a mixed failure mode are described by the percentage of each type of failure mode that is seen.

(15) TABLE-US-00003 TABLE 2 testing on oily cold rolled steel substrates Ex. or %- Lap Comp. HEM.sup.1 Shear Lap Shear Impact Sample capping, Str. Failure Peel Str. Impact Peel No. mol-% (MPa) Mode (N/mm) Failure Mode C-A 0 31 .sup.CF.sup.2 41 18%CF/82%BF 1 1.5 31 CF 43 67%CF/33%BF 2 5 28 CF 45 60%CF/40%BF 3 10 29 CF 36 35%CF/65%BF 4 20 26 CF 31 37%CF/63%BF 5 50 17 .sup.BF.sup.2 0.4 BF 6 80 11 BF 0.4 BF 7 90 11 BF 0.5 BF 8 95 10 BF 0.6 BF 9 98.5 11 BF 0.4 BF C-B 100 10 BF 0.3 BF .sup.1HEMA is 2-hydroxyethylmethacrylate.

(16) The data in Table 2 indicates that on an oily cold rolled steel substrate, the presence of some phenol capping in all cases increases impact steel strength (relative to Comparative Sample C-B). Lap shear strength is increased relative to Comparative Sample C-B in all cases in which 50% or more of the terminal isocyanate groups are capped with the phenol.

(17) Cohesive failure mode is improved in the lap shear test, relative to both Comparative Samples C-A and C-B, when from 1.5 to 20% of the terminal isocyanate groups on the toughener are capped with 2-hydroxyethylmethacrylate.

(18) In addition, impact peel strength is increased dramatically relative to Comparative Sample C-B when from 1.5 to 20% of the terminal groups on the toughener are capped with 2-hydroxyethylmethacryate. The impact peel strength is also better than in Comparative Sample C-A, when from 1.5 to 5% of the terminal isocyanate groups on the toughener are capped with 2-hydroxyethylmethacrylate.

(19) When evaluated on a 0.8-mm oily hot dip galvanized steel substrate (H340 LAD +Z, MCO 3028 oil), and on an a 1.3-mm aluminum (AC 120, surface pretreated with Bonder 299) substrate, Adhesive Examples 1 and 2 perform very similarly to Comparative Adhesive C-A in both lap shear and impact peel strength testings.