Coupling Agents for Adhesive Systems Patrick E. Cassidy,’ James M. Johnson, and Gary C. Rolls Tracor, Inc., 6500 Tracor Lane, Austin, T X 78711
A series of sixteen coupling agents was tested in urethane and epoxy adhesive systems as integral components of the mixtures. Lap shear tests were run on mild steel, aluminum, stainless steel, and glass cloth substrates at test temperatures of 196’, + 2 4 O , and +93’C. T-peel tests were also run at these three temperatures by use of aluminum panels with urethane adhesive. The additives were selected from silanes, phosphorus esters, chromium complexes, and cyclic and aliphatic amines and were screened at room temperature in the adhesive in concentrations up to 25%. The optimum concentration was then used for testing at high and low temperatures. The most consistently beneficial agents for the urethane adhesive were the epoxy-terminated alkoxy silanes whereas chloro- or mercapto-alkylalkoxy silanes and phosphorus esters occasionally increased the bond strength. Improvements by as much as lOOyo in bond strengths were accompanied by better reproducibility in a test series. However, for the epoxy system, additives were less effective and even were frequently detrimental to adhesion. Improvements were generallly less than 4oy0 with only one example as high as 70%. Beneficial agents included alkyl chlorosilanes, amines, a chromium complex, and one phosphorus ester.
-
Improvements in the total adhesive strength and in aging resistance are advantages in the use of coupling agents as adhesion promoters. It has been commonly postulated throughout the literature that this is due to primary bonds formed by the coupling species between the substrate surface and the organic adhesive. Other factors are also important, of course, such as wetting (Schrader, et al., 1967; Zisman, 1969), interlayers (Kenyon, 1968), and bulk properties. The bonding created by a n adhesion promoter occurs by virtue of its having functional groups which can react with the substrate and the organic phase. The most common case, silanes on glass, has been presented as forming siloxane bonds from the reactant to the glass surface and organic (covalent) bonds from the agent to the adhesive phase. This phenomenon then provides not only stronger interfacial primary bonding but also a condition not easily disturbed by environmental conditions-water absorption (DeLollis and Montoya, 1967).
0
Si
0
I
Si
0
I
Si
I I
Si
organic phase glass phase
Numerous agents are commercially available and are given in Table I according to type; those used in this study are given a designation. By far the most common are silanes which usually contain a n organic function and one to three hydrolyzable moieties (methoxy, ethoxy, or chloro). The mechanism of reaction then is for the water-sensitive function to react to form a silanol which then condenses spontaneously either with itself or a substrate to form siloxane (Sterman and hlarsden, 1963). The result is a polymerization process Present address, Department of Chemistry, Southwest Texas State University, San Marcos, TX 78666. To whom correspondence shoiild be addressed. 170 Ind. Eng. Chem. Prod.
Res. Develop., Vol. 1 1 ,
No. 2, 1972
which leaves a monomolecular crosslinked layer (“interphase”) on the surface (Kenyon, 1968; Matonis, 1969; Michelin, 1962). That a polymer is the absorbed species enforces the absorption strength of this phase-when a molecule is attached at many sites, it is much more difficult to desorb totally than individual monomeric species would be (Yates and Trebilcock, 1961). A second type of reactant is the chromium complex (Hauserman, 1959). I t s behavior and mechanism are similar to those of the silane in that metal-oxygen bonds are formed to the surface and within the agent layer after hydrolysis. It also possesses a n organic function (vinyl or alkyl) to bond to the organic phase with either primary or secondary bonds. The tetralkyl titanates are a third group which offer the same possibilities and are helpful in bonding thermoplastics (Gray et al., 1961). Finally, phosphorus compounds are beneficial coupling compounds when preapplied to glass (Schrader et al., 1967). They form primary bonds to glass surfaces, but they act only as monomeric species. These materials have also been grafted to thermoplastic surfaces to give a n improvement in adhesion. A review of all of these types of chemicals with a comprehensive bibliography has been prepared (Cassidy and Yager, 1971). For the study reported here, a n attempt was made to select a wide variety of additives so that an understanding of their behavior could be derived based on structure. A total of 14 agents was selected and used in each system. However, in some cases compatibility problems did not permit incorporation of the material into the adhesive. Two compounds have been included as possible coupling agents because of indications of chemisorption on steel by piperidine functions (Hackerman et al., 1962; Annand e t al., 1965). This work showed that the piperidine moiety was chemisorbed to a n iron surface, thereby performing as a corrosion inhibitor. Furthermore, poly(vinylpiperid1ne) was much more strongly chemisorbed than the monomeric species. It was therefore reasoned that an amino-substituted piperidine would have the ability to react with a n epoxy resin and pro-
Table I. Coupling Agents Type
Designation
Nomenclature and structure
Silanes R-Si3 (X = hydrolyzable group) Saturated alkyl and haloalkyl R group Methyltrichlorosilane CH3SiCl3 E th yltriethoxysilane CH3-CH2-Si(OEt)3 -pChloropropyltrimethoxysilane C1- (C Hq) 3-S i (0C H3) 3 Dimethyldichlorosilane (CH3)2SiC12 -4ryl and haloaryl R group Phenyltrimethoxysilane C&-Si(OC&)3 p-C hlorophenyltrichlorosilane C1-coH4-SiC13 Unsaturated alkyl R group Vinyltriethoxysilane C H F C H - S i (OE t) 3 yMethacryloxyprop yltrimethoxysilane
MTCS
CPS DMDCS
PTES
VTES MAPS
Type
Nomenclature and structure
Chromium complexes (Acid functions given) Methacrylo chromic chloride HzC=C(CHa)-COz-Stearo CHa(CHq)v,-COr-
Phosphorus compounds (R groups given) Phosphates Monoalkyl acids
0
I I/
R-0-P-
CH2=C-C-O-(CH2)3-Si(OCH3)~ Epoxy R group GPS
II
(OH)
Butyl Ethyl Isoamyl 0
/\
II
H~C-CH-CHZ-O-(CHZ)~S~(OCH~)~ ~-(3,4-Epoxycyclohexyl)-ethyltrimethoxy-
ECES
silane
Amino R group 7-Aminopropyltrimethoxysilane H2N-CH2-CH2-CH2-Si(OCH~)3
X-B-(hminoethy1)-yaminopropyltrimethoxysilane HJ-CHzCHz-NHSi(OCH3)3
AEAPS (CHz)3-
0
Phosphonates R-P-(OR)z Diphenylphenyl Dimethylmethyl Diethylethyl Bis-(2-ethylhexyl)-2 ethylhexyl Dibutylbutyl Phosphites (R-O)zP-OH Dimethyl Diethyl Diisopropyl Dioctyl Trifunctional phosphorus ester (exact structure unavailable) Miscellaneous Amines 4-Aminometh ylpiperidine
I!
CH3-O-C-CHq-CHz-XH-CHzCHz-NH-(CH2)3-Si(OCH3)3 Mercapto R group 7-Mercaptopropyltrimethoxysilane HS-CH2-CHz-CH2-Si( CH3)3
MCrCl
Tetralkyl titanates (R-O)r Ti (R-0 groups given) Isopropyl (CH8)zCH-0n-B u t yl CHI- (CH2)a-02-Ethylhexyl CHI- (CH2),-CH (CHZ-CH~)-CH~-OCresyl CHs-C6Hd-O-
CH3 0
-&lycidoxypropyltrimethoxysilane 0
Designation
H e c H q -
PE
Ampip
iwq
NIPS
duce a cured system with pendant adsorbable groups which act as a coupling agent to improve adhesion. To differentiate between the effects of the primary amine and the cyclic secondary amine in 4-aminomethylpiperidine, a model compound was used for comparison, cyclohexyl amine. This material did not contain the cyclic nitrogen function and should not then display chemisorption or coupling to the metallic surface. It is interesting that nearly all of the preceding work in
Cyclohexylamine
CHA
the field of coupling agents has been done with glass surfaces and by pretreating the substrate with the coupling chemical. I n this and a preceding study (Thompson and Hill, 1967), however, the behavior of these promoters as intregal components of the adhesive was evaluated on nonglass surfaces (aluminum and steel). Also, the effects of the coupling agents a t test temperatures of -196O, +24', and +93"C were determined. It is not the purpose of this work to completely evaluate Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
171
Figure 1 . Lap shear test specimen mold
each agent for its optimum application. The results herein relate to a special use of these materials and are not a n endorsenieiit or criticism of any product or manufacturer. Experimental
Urethane Adhesive. T h e material selected and standardized for use in this part of t h e program was Adiprene L-lOO/MOCh (Du Pont). The resin was mixed with t h e additive a t room temperature and with 0.2 wt yo glass beads (for bondline control). A quantity of MOCA equivalent to 12.5 rvt % was melted above 121°C and carefully poured and mised into t h e resin (the latter still a t room temperature). After thorough mixing with a spatula, the adhesive was applied to t h e coupons and cured a t room temperature for 16-24 h r , then a t 71°C for 24 hr. The coupons were then tested following any necessary flash removal. Degassing was unnecessary and in many cases actually decreased the bond strength and reproducibility. Epoxy Adhesive. The epoxy system was Shell Epon VI11 (Hysol Division, Adhesives Department) with 6 wt % Curing Agent "A" The resin was mixed with t h e coupling agent and 0.2% glass beads (2.4-3.5 mil diameter for bondline thickness control). The curing agent was then added and mixed thoroughly. The adhesive was applied to both surfaces of t h e bonding area of the coupons a n d was cured a t 93°C for 90 min. The hardened flash was removed with a small grinder. Bondline Thickness Control. Bond strengths are dependent on the thickness of the bondline; the type of relationship varies with the test method. Usually as t h e bond thickness increases, the strength passes through a maximum and then drops to a plateau (Shell Chemical Co., comn!ercial data; Thompson and Hill, 1967). I t is even possible t o change the failure mode with bondline 172 Ind. Eng. Chem. Prod.
Res. Develop., Vol. 1 1, No. 2, 1972
variations. Thin (2-10 mil) urethane bonds fail adhesively, b u t thick (I/* in.) ones fail cohesively. Therefore, to obtain meaningful data from lap shear and T-peel testing, i t was necessary to maint'ain a constant bondline thickness. For lap shear testing the desired thickness was approximately 0.003 in.; therefore, 0.2 wt % glass beads of diameter 0.0024 to 0.0035 in. were added and mised into the resin. A bondline t.hickness of about 0.007 in. was effected on the T-peel panels by using 0.2 wt % glass beads of 0.0058 to 0.0082 in. diameter (Thompson and Hill, 1967). A calculat,ion of the surface area of the glass beads in the test bond showed that area to be insignificant compared to the surface area of the substrate involved. Substrates. Preparation of t'he bonding surfaces was also well defined and controlled throughout t h e test program. For aluminum this consisted of wiping with trichloroethylene, vapor degreasing with t'he same for 10 rnin a t 8 i " C , etching for 20 min in a bath at 66°C (composition: 65.4 wt % water, 26.9 wt % sulfuric acid, a n d 7.7 wt % ' sodium dichromate dihydrate), washing with t a p water and then distilled water, drying a t 66°C for 10 min, a n d storing in a desiccator until use. It. was important t o use fresh solution in this etching process; therefore, 2.5 liters of solution were used for 64 coupons and then discarded. The mild steel substrates were wiped and vapor degreased with trichloroethylene, sanded with #l80 grit paper, degreased a second time, and used immediately. Stainless steel test specimens were washed with trichloroethylene, cleaned for 10 min a t 71" to 82OC (cleaning solution: 3 parts sodium metasilicate, 1.5 parts Xlconos, and 138 parts distilled water), et,ched in concentrated hydrochloric acid for 10 min a t room temperature, rinsed in distilled water, oven dried a t 66OC for 10-15 min, and used immediately. The glass cloth (used only with urethane adhesive) was # l l 6 with a VOLXS finish. To remove the finish, the samples were heated a t 450°C for 24 hr, then wiped, and vapor degreased with trichloroethylene for 20 min prior to bonding. Test Methods and Specimens. The method for preparing lap shear test specimens is shown in Figure 1. The lower two-finger pieces were covered ivith adhesive over t h e 1/2-in. faying surface and fitt'ed into t'he guide pins on the lower step of the mold. Adhesive was also applied t o the upper bonding piece and was laid over the first one to fit t'he pins o n t h e higher step. Then the bar, springs, a n d wing nuts were applied to maiiitain equal pressure during cure. On removal from the mold, the fingers were sawed t o give two tensile specimens or four per mold. The coupons were tested in tension on an Instron tester a t a crosshead speed of 0.05 in./min or approximately 1400 psi/min according to the procedure outlined in hSTMD187i-16T. After failure the bondlines were examined for degree of cohesive or adhesive failure and were primarily adhesive. The T-peel panels were bonded as 9 X 12-in. panels and then cut into 1 x 12-in. specimens with 3 in. of one end remaining unbonded so that it could fit into the jaws of the Instron to form the initial "T." The specimens were pulled a t a rate of 10 in./'min as suggested in XSThI-Dl877-16T. Cryogenic Tests. Although the high-temperature ( 9 3 O C ) t'ests could be run in the Instron environment chamber, a special holder was necessary for cryogenic ( - 196°C) test,s t,o allow rapid and accurate results. Figure 2 is a representation of this device which consists of a tall polyethylene or thin steel cup insulated with polyurethane foam. Through the bottom is projected a bolt which
Table II. l a p Shear Strengths (psi) for Agents in Urethane Adhesives" on Four Substrates at Room Temperature Additive, l.Owt%
Aluminum
None (control) DMDCS MTCS AEAPS
988 732 603
1525 f 189 1300 f 432 1017 f 137 Incompatible 2166 f 406 1905 =t481 1283 f 201 1385 f 215 1408 f 358 1802 f 372 1568 f 178 1571 f 169 1820 f 290 Incompatible 977 f 53
GPS CPS VTES PTES MAPS ECES MPS MCrCl
PE Ampip CHA 0
Mild steel
166 150 f 69
f
f
1545 f 213 1049 f 125 914 f 84 981 f 113 1010 f 216 1096 f 272 829 =t 31 983 f 57 847 f 145 921
f 33
(Aluminum carrier) glass cloth
Stainless steel
1130 994 222
f 229 =t144 f
1400
68
1220 f 165 1150 f 145 879 f 243 798 f 258 1040 265 1242 f 182 1446 f 259 1059 i 151 913 f 107
1499 f 125 135 1301
*
*
688
216
Z!Z
1237 f 133 134 1286 211 1497
* *
f 378
1211
&
245
865
&
149
Each value is an average of at least four specimens tested.
n
3/8" BOLT INTO BRASS BASE
1 11
(-si
F
B
R
A
S
S BASE
(It" D l A M E l E R )
U Figure 2. Cryogenic chamber
fastens to t h e lower jaw of t h e Instron. Fastened t o t h e bolt within t h e cup is a stainless steel plate which holds the sample in tension. To fit this particular sample holder, one end of the specimen was fitted with doublers (using the same adhesive as the test bond), and a hole and bolt were placed through these. This arrangement, shown in Figure 3, allowed facile placement of samples act,ually into the liquid nitrogen which promoted rapid cooling and provided excellent temperature control. On fracture of the bond, the lower piece
Figure 3. Cryogenic test specimen
was removed and another test specimen inserted and readied for test in a short time. Results and Discussion. Part I: Urethane Adhesive
Before the test program was instituted, all systems with no additives were tested to provide reproducible and optimum values. Here, the aforementioned procedures for sample preparation were refined to give maximum strengths, and these processes were followed throughout the program. Periodic checks were made to ensure the control values. Lap Shear Tests at Room Temperature. UrethaneAluminum System. Table I1 shows t h e complete results of coupling agent screening with the aluminum-urethane system. These data show that four commercial agents conInd. Eng. Chem. Prod. Res. Develop., Vol. 11,
No. 2, 1 9 7 2
173
Table 111. Agent Optimization in Aluminum-Urethane System at Room Temperature Additive and concn, %
Av shear strength, psi
GPS 1 3 5 10 15
2166 + 406 1838 f 318 1331 f 225 1461 =k 61 1478 f 254
Table IV. Agent Optimization in Mild Steel-Urethane System at Room Temperature Additive and concn, %
GPS 1 3 5
10 15
Av shear strength, psi
1545 f 213 1701 f 275 1649 f 481 1102 f 138 1100 f 368
CPS 1
3 5 10 15 ECES 1 3 5 10 15
1905 f 481 1741 f 173 1490 f 92 1669 f 217 1445 i 109
1802 i 372 1837 i 557 1841 f 545 1477 f 289 1321 ?= 161
Table V. Agent Optimization in Stainless Steel-Urethane System at Room Temperature Additive and concn, %
Av shear strength, psi
MPS 1 3 5
1446 i 259 1045 i 121 1131 139
*
PE 1
3 5 10 15
1820 f 290 1875 150 1304 i 160 1346 f 250 1578 f 450
*
tribute to the adhesive strength of urethane on aluminum by increases of 20-40%. These materials (GPS, CPS, PE, and ECES) were thus chosen for testing a t -196" and +93"C. A\lso,from these data, some agents even decrease significantly the bond strength of the urethane adhesive. Of course, the amine-terminated additive (AEAPS) is not compatible with the isocyanate resin owing to the high reactivity of the two. Of the four final additives selected, two are epoxyterminated methoxy silanes (GPS and ECES), one is a phosphorus ester whose exact structure is unknown (PE), and one is a chloroalkyl methoxysilane (CPS). It appears then that the methoxysilane is certainly applicable to a urethane system as is a n epoxy-terminal function. Table I11 shows the results of concent,rationvariation of the above chosen four coupling agents in the urethane-aluminum system. These data show that a sufficient and optimum concentration for the use of these agents is near l.O%, the composition of the material notwithstanding. As a result, all further testing with these adhesion promoters was conducted a t 1% concentration. Possibly, lower levels would be beneficial, but preliminary tests indicated that below 1%, differences are slight for urethane (Thompson and Hill, 1967). Crethane-Mild Steel System. Table I1 shows the results of additive screening with the mild steel-urethane system with additive concentrations of 1.0%. Five commercial agents contributed somewhat to the adhesive strength on steel; however, only one of the materials (GPS) demonstrated a significant benefit. The reactants most beneficial in this syst'em were also the only epoxyterminated compounds tested (GPS and ECES). Table IV shows the effect on adhesion of varying the concentration of GPS from 1-15% in the urethane-steel system and that 3% gave maximum benefit. Crethane-Stainless Steel System. I n Table I1 are the effects 174 Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
of adhesion promoters on the urethane-stainless steel system. Only ;\!IPS, a mercapto functional methoxy silane, proved to be beneficial, and this agent was selected for concentration variation testing. These data are shown in Table V. Since adhesive strengths fell off rapidly when more than 1% of these materials was used, testing was not pursued past the 5% agent level. Urethane-Glass Cloth System. For this particular substrate, a modified sample preparation was required. A sample of glass cloth was placed in the bondline of a usual lap shear bonding specimen. Of course, to obtain a valid test, the failure must be adhesive from the glass; therefore, a high urethanemetal bond strength is required. Because of its higher lap shear bond strengths in the urethane system, aluminum was chosen as the "carrier" substrate for the glass cloth. Both in baseline determination and agent screening in Table 11, adhesive failure of the urethane from the 116 glass cloth occurred. I n the case of CHA, a combination of adhesive failure from both glass and aluminum substrates occurred. Only those agents beneficial in the urethane-aluminum system were screened in the urethane-glass system. Only GPS and LIPS gave any increase a t all from the control value, and this difference was so small it was within the experimental error. Therefore, further testing was not pursued for this system. Tests at Extreme Temperatures. After the beneficial additives were selected by screening a t room temperature (at 1% concentration) and after the optimization of those agents, a testing program ensued t o determine t h e agent effect a t both high and low temperatures. Table V I gives t h e results of these tests. At +93"C for aluminum substrates, three compounds gave nearly a 100% increase in strength over the control value. These benefits were more pronounced a t this high temperature than a t room temperature. However, those which performed best a t +24"C also were best a t +93"C. These three agents were either chloro- or epoxy-alkylalkoxy silanes or a phosphorus ester. The remaining agent also contained the epoxy terminal function to confirm the utility of such a group in a urethane system. At cryogenic temperatures all bond strengths increased drastically (400% of room-temperature
Table VI. Substrate
Aluminum
Mild steel Stainless steel
Compilation of Averages from l a p Shear Testing in Urethane Adhesive Coupling agent and concn, %
- 196°C
Room temp
+93oc
Control 1 GPS 1 CPS 1 ECES 1 PE Control 3 GPS Control 1 MPS
5023 f 657 8497 f 403 7997 f 683 8414 f 874 6943 f 873 4913 f 607 11088 f 908 1012 4388 7187 f 1413
1525 f 189 2166 f 406 1905 f 481 1802 i 372 1820 f 300 988 f 166 1701 f 273 1130 f 229 1446 f 259
545 f 61 997 f 351 1062 f 75 881 f 177 1059 f 271 878 f 168 1074 f 196 510 f 104 741 f 162
*
Table VII. Agent Screening in 1-Peel on Aluminum at Room Temperature
Table IX.
Average l a p Shear Strengths (psi) of Epoxy Adhesive with Coupling Agents
Coupling agent, 1 wt%
Av peel strength, piw
Additive, 1 .o wt
Aluminum
Control GPS CPS PE ECES PE
38.2 65.1 38.0 -8 60.9 -6
None (control) DMDCS MTCS AEAPS GPS CPS HFS-1' HFS-20 VTES PTES MAPS ECES M PS MCrCl PE Ampip CHA
3150 f 190 3212 f 128 2983 + 117 3156 f 60 3212 f 72 3126 f 134 3102 f 246 3293 i 257 3243 f 143 3238 f 166 3183 f 103 3118 f 74 3209 f 151 3198 f 106 3392 i 148 2995 i 145 3198 f 58
Table VIII. Temp, 'C
96 96 96 - 196 - 196 - 196
T-Peel Testing on Aluminum Coupling agent, 1 wt%
Av peel strength, piw
Control GPS ECES Control GPS ECES
19.8 41.7 27.4 -9 -6 -7
values), partially because the urethane was in its glassy state. However, improvements over the control value were similar a t either temperature. On a mild steel substrate only one material showed sufficient benefit to allow further testing. This glycidoxy-terminal silane improved the strength by only 25% a t 93°C but by 126% a t - 196OC. This low-temperature bond strength of 11,088 psi was the highest realized in this program. On stainless steel the increase in strength offered by the mercaptoterminated silane was moderate a t +93" (45%) and at - 196OC (65%). T-Peel Tests. Table VI1 shows t h e result of screening by T-peel those agents which were beneficial in the aluminum lap shear tests. GPS, 1%, increased T-peel strength by approximately 60%. CPS, 1%, had no effect on T-peel strength whereas PE, 1%, essentially destroyed T-peel strength. This latter effect was a reproducible one. Table VI11 shows the results of high and low-temperature testing in the T-peel system. As a t room temperature, 1% concentrations of GPS and ECES substantially increased T-peel strength a t +93"C. At both +24" and +93OC, the test samples suffered total adhesive failure. However, a t - 196OC, the urethane was cooled through its glass-transition temperature and far into the glassy state and suffered total cohesive failure. Consequently, the resulting T-peel strengths
%
a
Mild steel
Stainless steel
2240 i. 222 3631 f 237 3206 f 86 2321 f 55 2343 i 243 3401 i 213 1794 f 338 3248 56 2864 f 496 2036 f 456 1487 f 297 1968 f 242 3684 f 204 2105 f 179 2278 f 70 3487 i. 209 3237 f 119 1797 f 353 1780 i 268 2939 f 183 2913 f 209 1805 f 383 3140 i 24 1857 f 245 1786 f 214 2905 h 169 1709 i 129 4000 i 70 2296 f 212 2894 i 38 2717 f 77 2341 f 81 3003 f 223 2344 f 58 Hydroxy functional silane, exact structure unknown.
were low and did not reflect the resin-substrate coupling ability of agents involved. When a substance forming the bondline achieves a high modulus by becoming glassy, i t is no longer able to relieve the high-stress concentrations encountered a t the edge of the bondline, and it fails catastrophically a t relatively low-tensile values. Results and Discussion. Part II: Epoxy Adhesive
This phase covered only lap shear methods since the brittleness of the adhesive did not allow reliable data in T-peel. Also, glass cloth was not used as a substrate because cohesive failure predominated. Room Temperature Testing. Epoxy- Aluminum System. Given in Table IX are t h e lap shear test results cf t h e 16 commercial materials inherently mixed in Epon VI11 followed by t h e addition of catalyst. Upon investigation of t h e aluminum d a t a , t h e improvement obtained by incorporation of 1% of adhesion promoters in epoxies is small (8yo increase in the best case). I n this initial screening, the most beneficial agents were PE and HFS-2 which were selected for more extensive testing. Unfortunately, the exact structure of neither of these materials is known with certainty. Earlier in this paper, it was confirmed that a concentration of 1% of the coupling agent in a urethane adhesive was near Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1, No. 2, 1972
175
Table X. Aluminum-Epoxy Effect of Additive Concentration Additive concn,
%
None (control) 1 PE 3 PE 5 PE 10 PE 15 PE 25 P E 1 HFS-2 3 HFS-2 5 HFS-2 10 HFS-2 15 H F S S
Av shear strength, psi
3150 i 190 3392 f 43 3313 f 130 3142 i 130 3510 f 110 1869 f 539 495 f 165 3293 i 257 3242 =k 150 3367 221 3610 f 182 2195 f 947
Table XI. Mild Steel/Epoxy Effect of Additive Concentration Additive concn,
%
None (control) 1 DMDCS 3 DMDCS 1 Ampip 3 Ampip 5 Ampip 10 Ampip 15 Ampip 1 CHA 3 CHA 15 CHA 1 MTCS
Av shear strength, psi
2240 f 222 2321 f 55 2526 i 110 2341 i 81 2332 f 36 2110 f 302 2293 f 93 2609 f 19 2344 + 56 2598 30 1942 f 1002 2343 f 243
*
Table XII. Stainless Steel/Epoxy Effect of Additive Concentration Additive concn, %
None (control) 1 MCrCl 3 MCrC1 5 MCrCl
Av shear strength, psi
3631 i 237 4000 =t70 3172 f 164 2818 f 150
optimum. However, since no data were immediately available concerning the optimum agent concentration in epoxy systems, a brief study was made. Table X gives the data obtained by varying the concentration of those two additives which demonstrated greatest improvement of lap shear adhesive strength in the epoxy-aluminum system. The optimum concentration of 10% was then selected for further testing at high and low temperatures (to be discussed later). Obviously, the necessary concentration of agents in the epoxy-aluminum system is not 1% but varies substantially. This, of course, means that the screening process conducted at the level of 1% additive may leave some beneficial materials unnoticed. PE was the only substance tested a t 25% concentration since this level was suggested in the manufacturer's literature. Epoxy-Mild Steel System. Table IX shows the results of the commercial agents used as additives a t 1.0 wt % concentration in epoxy on a mild steel substrate. Only four (DMDCS, MTCS, Ampip, and CHA) gave any substantial improvement in lap shear adhesive strength. Both DMDCS 176
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 2, 1972
and MTCS are alkyl chloro silanes, and the other two additives are either primary or cyclic secondary amines. Many additives seriously decrease adhesion when incorporated into the resin in this particular system. Shear strength values for mild steel bonded with Epon VI11 adhesive with varying concentrations of CHA, DPVIDCS, Ampip, and MTCS are given in Table XI. Concentrations of DMDCS above 3% (5,10, and 15%) and MTCS above 1% in Epon VI11 inhibited polymerization to such a n extent that the test bonds did not cure to hardness after 90 min at 93"C, the standard cure schedule. Even the 3% concentration of DMDCS eventually proved unreliable because of its proximity to the inhibition level. For this reason the 1% concentration of DMDCS was chosen for further testing in this system a t the extreme temperatures discussed later. Ampip was tested a t seven concentrations, 1, 3, 5, 10, 15, 20, and 25%. Because i t had the unusual effect of increasing shear strength even a t a 15% concentration, the higher concentrations of 20 and 25% were tested but shortened pot life too much to allow bonding. Impressive shear strengths were obtained with the substituted piperidine while permitting the resin to cure more easily a t room temperature. Reproducibility in this system is excellent (f0.5% a t 15% of Ampip). This increased adhesion and short-cure schedule may be caused by either the cyclic amino group or the primary amino function of the Ampip. To establish which of these moieties is responsible, a similar compound was used as a n additive, cyclohexylamine (CHA). This compound provides a primary amine as a reactive site but no cyclic secondary nitrogen. The results given in Table XI show that the CHh does not effect a n improved cure as does the piperidine. This would then infer that the bonding ability of Ampip is due to the primary amine, and the curing properties are due to the cyclic secondary amine. It then appears that chemisorption of the piperidine function is not a n important factor here. Epoxy-Stainless Steel System. Table IX gives the results of screening all additives in the epoxy-stainless steel system. MCrCl increased adhesive strength by the greatest amount; therefore, it was chosen for further testing in this system at extreme temperatures. Most other agents were quite detrimentral to adhesion. Data obtained from concentration variation of hSCrC1 (Table XII) indicate that a level of 1 wt % is most beneficial. The degree of cohesive failure in the epoxy-stainless steel system appeared to be 100% for the control samples and for those additives producing values equal to or greater than baseline shear strength. The increase in concentration of MCrCl did not affect the type of failure (cohesive failure was retained) but rather apparently affected the bulk properties of the resin in a n adverse manner. This brought about cohesive failure a t l o a shear strength. An interesting phenomenon which prevailed throughout this program was the frequent improvement of data reproducibility when a beneficial additive was used. TVhen improvement in bond strength &-as realized, it was usually accompanied by a smaller spread of data for the four (or more) samples. I n some instances the deviation n-as decreased from 1 1 0 to i0.5yG.This would then indicate that an additional benefit of the coupling agents is the increased reliability of the bond line. Another possible advantage of coupling agents is to improve the retention of bond strength with aging. This phenomenon would not necessarily be apparent in the data given here since it is not directly related to initial bond strength.
Table XIII. Compilation of Averages from Comprehensive l a p Shear Testing of Epoxy Adhesive Substrate
Aluminum Mild steel
Stainless steel
Coupling agent and concn, %
Control 10 HFS-2 10 PE Control 1 DMDCS 1 MTCS 15 4-Ampip 3 CHA Control 1 MCrCl
- 196’C
Room temp
1562 f 354 1984 f 392 1448 f 218 1042 + 158 1194 f 136 1438 f 136 845 i: 138 1293 f 373 1610 f 54 1399 i: 299
3150 190 182 3610 3510 =k 110 2240 f 222 2321 f 55 2343 + 243 2609 f 19 2598 f 30 3631 + 237 4000 i: 70
Extreme Temperature Testing. T h e screening results in Table I X show t h a t seven agents were effective in improving lap shear strength, dependent on substrate. After t h e optimization of each of these according t o concentration, they were subjected t o testing a t -196” a n d +93”C. Table XI11 gives t h e summary of these results. High-temperature results on aluminum indicate t h a t all of the additives are harmful to adhesion, and cohesive failure prevails. The reason for this is not immediately obvious since some improvement is realized a t room-temperature testing, and the high-temperature testing does not occur above the cure temperature used. One possibility is that the additive causes the glass transition (Tg) of the system to be decreased in the system to much less than 93°C. This would mean that with additives, the epoxy a t 93°C is being tested when its amorphous phase is farther into the rubbery state than the control resin. I n considering the chemical composition of the additives (a hydroxy functional silane and a phosphorus ester), this phenomenon is understandable. These materials may react with the resin to reduce its functionality, thereby decreasing the crosslink density. If this reactivity and subsequent decrease in crosslink density are realized, a significant change in bulk properties will in turn be manifested in the thermal behavior of the resin. This phenomenon has been discussed more extensively (Cassidy et al., 1972). At low temperatures the hydroxy silane does improve strength by 27% although the phosphorus ester is somewhat harmful. The PE material was expected to give better results, since it has been described as a flexibilizer and should therefore contribute to low-temperature properties. Throughout the epoxy data, testing at one temperature is not a good indication of relative strength over a wide temperature range. O n a mild steel substrate, 15% Ampip demonstrated the greatest improvement of shear strength at room temperature, but it essentially destroyed shear strength at both +93” and - 196°C. M T C S demonstrated a remarkable ability to retain room temperature strength a t the higher temperature and has been the only additive a t any concentration in any system to demonstrate that ability on a n y substrate. Once again i t was not the best performer at room temperature; therefore, relative strengths a t one temperature cannot be considered the same under other conditions.
+93oc
*
2621 567 1480 1382 1540 2378 331 1877 2570 2242
f
137
i: 59 f 284
i: 280 f 140 f 110 f 64 f 547 f 362
f 494
On the stainless steel substrate the same detrimental effect of the agent is observed a t both high and low temperatures, as was the case for aluminum. Whether this effect is due to the metallic surface or to the agent/resin interaction was not established. Acknowledgment
The authors wish to thank NASA personnel (George C. Marshall Space Flight Center, Huntsville, AL) Leldon Thompson and W. E. Hill for their imput to this program. Also gratitude is due B. J. Yager, Professor of Chemistry, Southwest Texas State University, San Marcos, TX, for his contributions. Literature Cited
Annand, R . R., Hurd, R. M., Hackerman, N., J . Electrochem. SOC.,112 (a), 138 (1965). Cassidy, P. E., Yager, B. J., J . Mucromol. Sci., Rev. Polym. Tech., DI, 1 (1971). Cassidy, P. E., Johnson, J. M., Locke, C. E., J . Adhes., in press (1972).
DeLollis, N. J., Montoya, O., J . A p p l . Polym. Sci., 11, 983 (1967).
Gray, Jr., c. L., MacCarthy, W. L., McLaughlin, T. F., Mod. Packag., 34 (lo), 143, 243 (1961). Hackerman, K,,Hurd, R. M., Annand, R. P., Corrosion, 18, 37t (1962).
Hauserman, F. B., Advan. Chem. Ser., 23, 338 (1959). Kenyon, A. S., J . Colloid Interface Sci., 27 (4), 761 (1968). Matonis, V. A., Polym. Eng. Scz., 9 (2), 100 (1969). Michelin, Durin and Cie., French Patent 1,278,030 (1962). Schrader, M.E., Lerner, I., D’Oria, F. J., Xod. Plast., 45 ( l ) , 195, 268, 272, 276, 280, 282 (1967).
Sterman, S., Marsden, J. G., ibid., 40 ( l l ) , 125, 127, 129, 134, 136, 138, 177 (1963).
Shell Chemical Co., Adhesive Dept., commercial data. Thompson, L. M., Hill, W. E., “A Preliminary Evaluation of Silane Coupling Agents as Primers and Additives in Polyurethane Bonding Procedures,” NASA TM X-53676, George C. Marshall Space Flight Center, Huntsville, AL, November 28, 1967.
Yates, P. C., Trebilcock, J . W., Reinforced Plastics Div., Soc. of Plastics Inc., 16, 8-B (1961). Zisman, W. A., Ind. Eng. Chem. Prod. Res. Develop., 8 (2), 98 (1969). RECEIVED for review August 24, 1970 ACCEPTED January 26, 1972
This work was supported by the National Aeronautics and Space Administration, George C. Marshall Space Flight Center, Huntsville, AL, and was presented in part at the Xational SAMPE Technical Conference, Dallas, TX, October 6-8, 1970.
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