Ind. Eng. Chem. Prod. Res. Dev. 1085, 2 4 , 565-567 Stather, F.; Nebe, H. Gesammelts Abh. Dsch. Lederinsf. Freiberg 1951, No. 7, 34-47. Steindroff, A.; Balle, G. German Patent 365286, 1920. Strubell, W. J. Prakt. Chem. 1959, 9 , 159-169. Tallis, E. F. U.S.Patent 2716083, 1955. Turkington, V. H.; Butler, W. H. U.S. Patent 2017877, 1935.
565
Turklngton, V. H.; Butler, W. H. U.S.Patent 2 173346, 1939. Winguist. A. D., Jr. British Patent 1032 885, 1966.
Received for review February 4, 1985 Accepted May 21, 1985
Organophosphorus Enamines: A New Class of Epoxy Hardeners and Adhesion Promoters Milton D. Johnson,' Mohinder S. Chattha, and Richard E. Robertson Research Staff, Ford Motor Company, Dearborn, Mlchlgan 48 12 1
A new class of organophosphorus enamines that are useful as adhesion promoters for epoxy resins have been prepared. The enamines were synthesized by nucleophilic addition of aliphatic amines to the activated triple bonds of tris(pentyny1-1)phosphine oxide. These enamines react with epoxy resins to provide thermoset materials. Incorporation of these compounds into amine-cured epoxy formulations provides improved adhesion to glass under conditions of stress, high humidity, and elevated temperatures. Application of the organophosphorus enamines as primers on the glass surface also provided Improved resistance to adhesive failure under these conditions.
Introduction Incorporating a polar functional group, such as pentavalent phosphorus, into a polymer increases adhesion to polar solids such as glass, cellulose, and ceramic materials (Cassidy and Yager, 1971). Amines and other species having lone pairs of electrons are also known to coordinate strongly to polar surfaces (Sander and Steininger, 1967). A new class of epoxy curing agents containing both amino and pentavalent phosphorus moieties has been synthesized and investigated for use as adhesion promoters. This paper describes the improved adhesion of epoxy resins to glass attained by incorporating organophosphorusenamines into epoxy adhesive formulations. Since adhesive failure is most likely under conditions of high humidity and elevated temperature, all the materials were tested under these conditions. Experimental Section Materials. Pentyne, obtained from Farchan Chemical Co., was dried over molecular sieves before use. Butyllithium was obtained from Ventron. Amines were purchased from Aldrich Chemical Co. and were dried over molecular sieves. Reagent-grade phosphorus oxychloride was further purified by distillation. The epoxy resin employed was bisphenol-A diglycidyl ether, DGEBA, from Shell Chemical Co. Analysis. Infrared spectra were recorded on a Perkin-Elmer 453 spectrophotometer and the NMR spectrum was recorded on a Bruker HX-60 spectrometer. Thermal analysis was performed on a Du Pont 950 thermogravimetric analyzer. Synthesis of Tris(pentyny1-1)phosphine Oxide (1). Anhydrous 1-pentyne (10.2 g, 0.15 mol) was dissolved in 750 mL of anhydrous diethyl ether and placed under nitrogen in a 2-L three-necked flask. The flask was cooled with dry ice, and a 2.4 M solution (62.5 mL, 0.15 mol) of n-butyllithium in hexane was added slowly with continuous stirring. The reaction mixture was stirred for 1h, and a solution of phosphorus oxychloride (7.7 g, 0.05 mol) in 100 mL of diethyl ether was added dropwise with continuous
stirring and cooling. After the addition was complete, the reaction mixture was allowed to warm slowly (1h) to 0 "C. It was stirred for an additional 0.5 h and was worked up by slowly adding 150 mL of cold water. The layers were separated, and the aqueous layer was extracted with two 50-mL portions of chloroform. The combined organic extra- were dried over anhydrous magnesium sulfate, and the solvent was stripped after filtration. The oily residue was short-path-distilled under reduced pressure to obtain 10.5 g (84%) of the desired product 1: bp 171-172 "C (0.1 mm); IR (Figure 1)2190 (CEC); NMR (CDC1,) 6 2.2 (m, 6, JHH = 7 Hz, J ~ =H 2 Hz, C=C-CH&, 1.68 (m, 6, J = 7 Hz, C-CH,-C), 1.08 (t, 9, J = 7 Hz, CH,). Synthesis of Tris(2-(n-butylamine)-1-penteny1)phospine Oxide (Adduct 2). Compound 1, 10 g, was dissolved in 100 mL of n-butylamine; even though an initial exothermic reaction occurred, the IR after 1 h of mixing showed that addition of amine to the triple bonds was incomplete. The mixture was refluxed for 1h and the IR then showed complete disappearance of the C=C absorption (2190 cm-l) and appearance of a very strong C = C absorption (1610 c m 3 . The excess amine was distilled off under reduced pressure by using a water aspirator, and its last traces were removed by using a vacuum pump to obtain the desired product in quantitative yield as a viscous oily material. Synthesis of Adduct 3. This material was prepared by treating 1 g of compound 1 with 4 g of diethylenetriamine (DETA). The initial reaction was exothermic; however, the addition went to completion only after heating for 1 h at 90 "C, as evidenced by disappearance of the C=C IR absorption. Epoxy Resin Cure with Enamine 2. DGEBA (equiv wt 185-192), 1.2 g, was mixed with 1.29 g of enamine adduct 2 (equiv w t 189) and placed in an oven at 125 "C for 1h. The mixture cured to give a hard transparent solid. Thermal gravimetric analysis of the solid material under nitrogen showed no weight loss until decomposition began near 260 O C (Figure 2), indicating that the material was well cured. 0 1985 American Chemical Society
566
Dev., Vol.
Ind. Eng. Chem. Prod. Res.
24, No. 4, 1985 STEEL
GLASS
Figure 3. Fracture energy test configuration. io 4000
I
I
3500
30W
I 2500
1
2000
I I800
WAVENUMBER
I 1600
1400
I 1200
I
IC00
KM"I
Figure 1. Infrared spectrum of tris(pentyny1-1)phosphine oxide.
I
100
i
200
300 T, C'
400
500
Figure 2. Thermal gravimetric analysis of DGEBA cured with adduct 2; heating rate, 5 OC/min, N2 atmosphere. Table I. Shear Strength of Bonds Exposed to 100% Relative Humidity at 60 "C curing agent time, h shear strength, kPa 22 000 DETA 0 24 6 200 DETA 4 800 48 DETA 22 000 0 adduct 3 24 15 200 adduct 3 48 13 800 adduct 3
Tensile Strength Determination. The tensile strengths of glass to steel bonds formed from DGEBA cured with adduct 3 were compared to those cured with DETA alone. Stoichiometricamounts of the curing agents, based on active amino hydrogens, were employed. The test samples were prepared by bonding annealed glass panels (76.2 mm X 25.4 mm X 4.76 mm) to sanded steel strips (10.2 mm X 6.4 mm X 0.66 mm) by curing the adhesive at 120 "C for 0.5 h; the bond area was 6.4 mm X 6.4 mm. Tensile lap shear strengths were determined by pulling the samples to bond failure at room temperature by using an Instron mechanical tester. The samples were aged in 100% humidity at 60 "C. The tensile strengths of samples before (initial strength) and after (final strength) aging are listed in Table I. Stress Corrosion Test. Test samples were prepared by using glass slides (76.2 mm X 25.4 mm X 1.22 mm) and steel strips (101.6 mm X 6.35 mm X 0.68 mm) cut from a 30.5 cm X 10.2 cm panel with Bonderite 37 treatment. One side and edge of the panel were marked before cutting with a shear; the strips were always oriented in the same manner. In this way, the variability due to slight metal deformation when the strips were cut was minimized. Three formulations were used A, DGEBA and DETA; B, adduct 2 as primer and DGEBA and DETA; and C, DGEBA and adduct 3. In all cases stoichiometric amounts of epoxy resin and hardener were used. For case B, prior to bonding, glass plates were dipped in a 1% solution of 2 in ethyl acetate. The plates were removed from the solution, and the solvent was allowed to evaporate at room
temperature for 1 h. The steel strips were bonded to the glass, leaving approximately 6.4 mm nonbonded length at the end. The samples were cured by placing them in an oven at 90 "C for 0.5 h. The excess adhesive along the edges of the metal was removed by using a razor blade. The thicknesses of the metal and glass were measured. A clamp was positioned 1.5 cm from the edge of the glass to prevent the crack from traveling too far when initiated; the glass and the metal were forced apart enough to insert a spacer 0.7 mm thick, as shown in Figure 3. The spacer was removed, and the distance from the bottom of the glass at the edge to the top of the metal was measured. Now the spacer was reinserted and the distance remeasured; the difference gave the effective thickness of the spacer. This method of measuring thickness, y, eliminated any variability due to bending that might have occurred. After the clamp was removed, the location of the crack tip was observed by holding the specimen between cross polaroids and observing the edge of the glass. The blue spot, due to maximum stress concentration, showed the end of the crack. The specimen was then immersed in water at 50 "C, and the length of the crack, c, was measured after various intervals of time. For a constant spacer height, y, the stresses at the root of the crack decrease as the crack grows. The amount of stress that the bond can withstand determines its resistance to cleavage. Hence, if after a certain length of time, t ', the crack length is found to be c ', the stresses existing at the root of the crack represent the resistance of the adhesive bond to failure in the time t'. Rather than computing the stresses at the root of the crack, an equivalent quantity, the fracture energy, was computed (Corten, 1972). For the test specimen, this is given by
EM[
fracture energy (G,) = 8
4 3 L
m
1+-
where El = modulus of steel, hl = thickness of the steel, E2 = modulus of glass, h2 = thickness of the glass, y = thickness of the wedged gap, and c = length of the crack. An advantage of translating the results of the cleavage test into the equivalent "fracture energy- is that the latter can be related through analysis of fracture mechanics to a maximum sustained bending moment. Results and Discussion Chemical Reactions and Structures. Pentynyllithium, formed by the reaction of butyllithium with 1pentyne, reacts with phosphorus oxychloride to produce tris(pentyny1-1)phosphine oxide (1) as shown in eq 1and 2. The infrared spectrum (Figure 1)of alkyne 1, showed C3H7C=CH + C4H9Li C3H,C=CLi + C4H10t (1) C3H7C=CLi + POC13 (C3H7C=C)3P=0 (2)
--
1
a very strong absorption for the C--1, (2190 cm-'), and the structure was further supported by proton NMR analysis. The alkynylphosphine oxide 1, when treated with primary aliphatic amines such as n-butylamine, produced
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 4, 1985 567
enamine 2 in essentially quantitative yield (eq 3). The (CSH~C=C)~P=O
t C4HgNH2
-
>;:.c[=cp].=o
(3) 3
2
addition of amine to 1 was followed by observing the rapid disappearance of C=C absorption (2190 cm-') and appearance of a strong C=C absorption band (1610 cm-') in the IR of the reaction mixture. The absorption band at 1610 cm-l supports the enamine structure rather than the isomeric azomethine structure bearing a C=N moiety (Chattha and Aguir, 1971,1973). Since the reaction was carried out in a large excess of amine, enamine is formed in essentially quantitative yield without any detectable heterocyclization taking place to produce phosphorylazacyclohexadienes (Chattha, 1980). When compound 1 is treated with an excess of polyfunctional aliphatic amine, such as diethylenetriamine, adducts like 3 are produced (eq 4). Some polymeric ad-
-
t HzNCH~CH~NHCH~CH~NH~
(C~H~C=C)SP=O
HeNCHzCHzNHCHzCH2NH\ C , =CH CsH7
L
P=O
(4)
J3
3
ducts may also be formed in the above addition reaction, since both the amine and the alkynyl phosphine oxide are multifunctional. Enamine adduct 2 reacts with epoxy resins to produce hard-cured materials. The major curing reaction taking place is epoxy ring opening by the imino function (eq 5).
-
t[ C4H9NH >C=gP=O C 3H7
3
2 -7
7
I YHI
OH
-CH-CHzN\
C4H9
II
C4Hg
/C=CH
I P=O
(5)
In the case of adduct 3, both amino and imino groups would participate in the epoxy curing reactions. Adhesive Performance. Fracture energies, G,, measured during stress corrosion testing of materials A, B, and C vs. time of immersion in water at 53 "C are shown in Figure 4. A comparison between these three plots shows that adhesive B (adduct 2 as primer) is significantly tougher than adhesive A (DGEBA cured with DETA). Similarly, adhesive C (DGEBA cured with adduct 3) has a higher initial G, than that of adhesive A and maintains a higher fracture energy, even after extended testing, than that exhibited initially by adhesive A. Thus, incorporation of organophosphorus enamines has provided thermoset epoxy adhesives that are superior in performance to those cured with amine alone. Tensile strength data (Table I) also support this conclusion, even though the initial strengths of the adhesives with and without the adducts were found to be the same. In all tests, both tensile and stress corrosion, the bond failure always occurred at the adhesive to glass interface.
0.2 I
5 IO TIME (days)
15 20
Figure 4. Fracture energy (G,)vs. time of immersion in water at 60 OC: (A) DGEBA cured with DETA; (B) adduct 2 as primer and DGEBA cured with DETA; ( C ) DGEBA cured with adduct 3.
Mechanism of Adhesion Enhancement. The semicoordinate P=O bond has been considered of significant importance in increasing adhesion to polar surfaces (Cassidy and Yager, 1971). Other factors to be considered are wetting, interlayers, bulk properties, absorbed species, environmental effects, etc. All of these parameters play important roles that cannot be clearly separated. The most distinctive feature of organophosphorus enamines is that they exist in cis and trans configurations (Chattha and Aguir, 1971,1973). In the cis configuration both the P=O and amino groups may coordinate with the same site to give a stable coordinate system. Even though this type of coordinate complex appears to be logical, it is also possible that the amino and/or P=O groups function independently, in which case the trans compound would also be effective. To convincingly demonstrate the mechanism of adhesion promotion by these organophosphorus enamines would require additional surface interaction and chelation studies. Conclusions Tris(pentyny1-1)phosphine oxide can be conveniently prepared by the reaction of pentynyllithium with phosphorus oxychloride in ether. Production of tris(enamines) by addition of aliphatic amines to tris(alkyny1)phosphine oxide is quite facile. These organophosphorus enamines react with epoxy resins to produce thermoset materials. Incorporation of these enamines into amine-cured epoxy adhesive formulations or application as a primer on the surface to be bonded provided bonds to glass with increased resistance to failure under conditions of high humidity and increased temperatures. €&&try NO. 1,98526-06-8;2,98526-07-9;DGEBA, 25085-99-8; LiC=C(CH,),CH,, 18643-50-0;POCI,, 10025-87-3;BuNH,, 109-
73-9.
Literature Cited Cassidy, P. E.; Yager. B. J. J . Macromol. Sci., Part D 1971, 7(1), 1. Chattha, M. S. Chem. Ind. (London) 1980, No. 4, 157. Chattha, M. S.; Aguir, A. M. J . Org. Chem. 1971, 36, 2892. Chattha, M. S.; Aguir, A. M. J . Org. Chem. 1973, 38, 820. Corten, H. T. "FractureMechanics of Composites",in "Fracture";Liebowietz. H., Ed.; Academic Press: New York, 1972; Voi. VII, Chapter 9. Sander, M.; Steininger, E. J . Macromol. Scl.. Part C 1987, 1(1), 1 .
Received for review February 11, 1985 Accepted May 22, 1985
