SOME CORRELATIONS AMONG THERMAL, ELECTRICAL, AND MECHANICAL PROPERTIES OF ALKYD-EPOXY COPOLYMER ADHESIVES . J. W A R D , dVew York State College of Forestry, Syracuse 70, K. Y. E . G . B 0 B A L E K , Case Institute o f Technology, Cleveland 6, Ohio R
1
All copolymers derived from uncatalyzed melt reactions of an alkyd with either of four aromatic polyepoxy intermediates showed peaks in lap-shear strength in aluminum alloy adhesive joints near their glass transition,
Tg. This critical region near Tg can b e determined from dissipation factor, measured either with the torsion pendulum on polymer rods, or nondestructively in adhered joints using a variable frequency, a.c., capacitance bridge. Although the latter gives values about 20" C. higher, either can b e used to follow cure rate. Large shifts of the Tg region can b e effected only b y changing copolymer composition. More or less drastic changes in curing schedule shift Tg only slightly; the main effect of overcuring is to increase fracture sensitivity, which narrows the temperature range near Tg where adhesive strength shows a maximum plateau.
the demands placed on the physical properties of structural adhesives, one of the most important is the retention of useful mechanical properties over a wide temperature range. Throughout the usable range of a n adhesive, the properties usually desired are high modulus and impact resistance. The lower end of the useful temperature range is often set by the temperature where the adhesive becomes brittle and loses its impact resistance, whereas the upper end is determined by a lack of creep resistance. Modern theory (24) recognizes a t least four reference temperatures within each thermal range Lvhich are related to the structure of amorphous polymer netivorks. For example? TO is the threshold of the glassy behavior. T Iis the midpoint of the transition region bet\veen glassy and rubbery behavior, and Tz. Ts?and Tq are? respectively, the midpoints of the regions of rubbery elasticity, rubbery flow? and viscous fluid flow. The over-all strength and stability of an adhesive joint are governed by a t least three major factors: viscoelastic behavior, adhesive failure near the interface, and aging stability. More often than not? adhesion forces are not the controlling factor of joint strength, as pointed out by Bickerman (4). Even in some cases where adhesive forces are deficient, ultimate joint strengths are determined by the properties of the bulk polymers. Therefore, a systematic approach to studying adhesives, based on corresponding states of viscoelastic behavior, may lead to the design of a n adhesive for a given thermal regime. Exploring the effect of variations of molecular architecture on adhesive quality ivithin a compositionally determined thermal range is the main concern of this study, particularly to determine whether or not optimum adhesive quality occurs in the glassy region, rubbery region, or, as suspected from previous work ( 3 ) .the transition region. For the main aspect of this study, a homogeneous alkyd-epoxy copolymer system was selected ivhich could be manipulated conveniently to reproduce all the above pattern of viscoelastic behaviors. F ALL
Present address, Department of Chemical Engineering, Univer-
sity of Maine, Orono, Maine.
To define the optimization temperatures and to correlate their results to the thermal-mechanical behavior of polymers, the strength of lap-shear joints was determined as a function of temperature and independent measurements \Alkyd 73-4 made from the recipe Mole 0.8 0.2 1 .0 0.4
Reactants Phthalic anhydride Trimellitic anhydride Glycerol Linoleic acid
has the following characteristics: Acid 2VO.
115 0
Hydroxjl
~VO. 171 8
DP, 4 46
Intrinsic Viscosity MEK DMF 0 0421 0 0422
hlelf ~ ~ s c o sL\foi, ~ ~ ~ , Poises LVt. 3 54 698
The epoxy resin intermediates chosen u e r e Shell Chemical Corp. Epon 828 [diglvcidyl ether of 2.2-bis(p-hydrox)phenyl)propane], Union Carbide Chemicals Corp. EP 201 (dicyclodiepox)carbouylate). and Dow Chemical Co. D E N 438 (epoxy novalac). VOL. 2
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Test Methods. LAP SHEAR. Overlap shear specimens were fabricated using aluminum adherends (1 X 0.064 X 4l/8 inches) of Alclad 24S-T3. The adherends with edges milled smooth were vapor-degreased with tetrachloroethylene and etched in a chromic acid bath (26). The specimens were prepared according to a modified version of ASTM Designation D 1002-53T ( 2 ) . Instead of making overlap sheets, each test specimen was made separately. The lap joint was initially made '/4 inch longer than the desired l/*inch overlap. T o obtain uniform thickness control, wire of the desired thickness was held in place between the specimens as shown in Figure 1. After curing, the specimens were kept in a dry C 0 2 atmosphere until tested. Prior to testing, the specimens were placed in a special jig and the wire was milled off, leaving the desired '/*-inch overlap.
WIRE 5 MILSXr,,,,,,
0.064" $ - - - - - - - - - - - - - - - - - -
I , L . - - / - / / / - - - - - - /
I+----
4 g
1-
t
Figure 1 . Method of controlling thickness of lap-shear joints, showing wire placement
The lap shear specimens were tested in tension with the use of an Instron testing machine modified with a Sanborn Recorder (Model 154-100 B) for high speed recording and equipped with a n oven which permitted testing over a wide range of temperature. T o be sure that test specimens were in equilibrium with test temperature, the samples were conditioned for a t least 15 minutes before breaking. TORSION TESTS.Castings were made from the copolymers described above to examine their low-shear dynamic properties. A schematic diagram of the free oscillation torsion pendulum used in this study is given in Figure 2. The polymer sample, F, is held in clamps, G, a t both ends. The samples used were approximately 2.65 X 0.25 X 0.12 inches. The width and thickness were measured to the nearest 0.001 inch and the length to 0.01 inch. The sample, clamps, and inertia disk were completely surrounded by a n insulated oven which could be cooled to temperatures as low, as -80' C. using dry ice and liquid nitrogen. Mechanical damping, represented by the logarithmic. 6, is calculated from the rate of decrease in twisting energy caused by mechanical loss in the form of heat. 6 is the natural log of the ratio of amplitudes, A , of two successive vibrations
where K is the number of periods separating the individual amplitudes considered. The shear modulus, G, was calculated from Equation 2 :
where C = width of specimens, inches D = thickness of specimens, inches L = length between clamps, inches I = moment of inertia, grams per sq. cm. p = shape factor (78) p = period of oscillation, seconds The data necessary to obtain 6 and G, other than physical dimensions, a t any given temperature are the successive amplitudes and the period of oscillation. ELECTRICAL TESTS.The first type of electrical specimen used was made like the lap-shear specimens, but the overlap was made so that a 1-inch square could be cut from the specimen. The specimens for thickness studies were made from 1-inch diameter stock approximately 1 1 / 2 inches long. The cylinders were faced and polished on a precision lathe and given the same etching procedure as the lap-shear specimens. The samples were cured in a jig which provided a means of setting a predetermined thickness. 86
l&EC P R O D U C T RESEARCH A N D DEVELOPMENT
I
I
Figure 2. pendulum
Schematic diagram of torsion A. B.
Counter weights Counter weight arm C. Support wire D. Indicator needle E. Inertia disk F. Plastic specimen G. Specimen holder H. Rigid mounting 1. O v e n bottom J. O v e n top
.411 electrical measurements were made with a General Radio Type 71 6-C capacitance bridge, a n internally shielded Schering hridge which has direct readings in capacitance and dissipation factor ( 7 ) . A cathode-ray oscilloscope was used to detect the bridge balance. The dielectric samples were held in a special spring-loaded clamp. The apparatus was enclosed in a polyethylene bag containing silica gel to prevent moisture condensation and the entire assembly was placed in a \vire cage which permitted the use of a General Radio Type 716-P4 guard circuit to eliminate stray capacitance from the measurements. The various resin systems used were investigated over wide ranges of temperature, and readings of capacitance and dissipation factor were taken every 5' to 10' C. From these data, plus the specimen dimensions, the loss factor was computed. I t was expected that this test \vould tell not only the location of a thermal transition of mechanical behavior but also how the orientation effects, "pinch effects" (25), etc., endemic to an adhesive joint influence the transitions. It was anticipated that \vith thick films. electrical and mechanical tests could be directly correlated. As the film thickness declines, the secondary orientations could have a significant influence on the apparent modulus and dissipation factor Results and Discussion
Adhesive Composition Determination. The more detailed part of this investigation involved the use of Alk>-d 73-4
copolymerized with one of the following epoxy resins: A. Epon 828 (copolymer with 73-4 called System A). /
0\
CHz-CH-CH2-0
0 CH3 / \ -(-)-C~O-CH~-CH-CHz -
1
-
CHI
B. DEN 438 (copolymer with 73-4 called System B). /O\ O-CH2-CH-CHz
L
?i
O-CH2
J n=1.3
-
CH-CH2
The optimum compositions for Systems .4and B were determined by mixing various ratios of the homopolymers and testing lap-shear strengths as a function of cure temperature. T h e results of System .4 are shown in Figure 3. T h e weight ratio of 2.3 to 1 (Alkyd 73-4-Epon 828) gave the strongest bonds. This optimum ratio is temperature-independent from 140' to 200' C. The reproducibility of the lap-shear specimens was within about +lo'%. Therefore, the apparent differences in maximum strength were not considered real, since they showed no systematic increase with cure temperature. Cure temperature above 160' C. produced a bubbling action which was probably due to a condensation reaction other than the opening of the oxirane ring. Because of this phenomenon, System .4\vas cured a t 140' C. The resin system \vas made more complex by using DEN 438 (System B) as the cross-linking agent. This higher functional polymer led to several changes from the former system. As shown in Figure 4, this system is no longer temperature-independent. As the cure temperature is increased, the optimum composition is one richer in DEN 438. This is probably due to a steric effect of the more bulky system. Effect of Curing Conditions. T h e theoretical shear strength of an adhesive bond as it varies with extent of cure is illustrated schematically in Figure 5 . O n e of the major factors in determining the structural strength of a polymer system is its molecular weight. A material of low molecular weight has a n insufficient number of secondary bonding sites between individual molecules to prevent their slipping past one another under a low stress. This is depicted by Region -4, the low molecular weight plateau. As the curing time is increased, the molecular weight increases, and if there is little or no cross linking, the individual chains grow longer. As this happens, the cohesive strength increases not only because of the greater number of secondary bonding sites per molecule: but also because of increased degree of entanglement of the longer chains. This effect is shown in Region B, the viscosity transition zone. T h e shear strength continues to increase until the cohesive strength between molecules becomes equal to the primary bond strength of the molecules. T h e curve then reaches a plateau, Region C, which is nearly independent of molecular weight and chain entanglement. If curing is continued beyond the threshold of the molecular weight-independence region, a loss in shear strength may be incurred by embrittlement of the film. This embrittlement could be caused by cross linking or phase separation of the homopolymers, or because cohesive forces and/or shrinkage in the polymer reduce the number or strength of adhesive bonds. T h e width of these regions will depend upon the extent of polymerization and local architecture of the polymer. The influence of thermal history in preparation of adhesive joints or in aging of adhesive joints should be rated with reference to effect on regions A, B, C, or D of Figure 5. To
1
,;::;
I
20 0
i
0
,I
2 3 4 5 I COMPOSITION RATIO (ALKYD /EPOXY 1
Figure 3. Composition of Alkyd 73-4-Epon 8 2 8 vs. maximum strength
I 1.0
I
0.5
I 1.S
COMPOSITION
I
I
L
I
2.0 2.5 3.0 RATIO (Alkyd/Epoxy)
3!
Figure 4. Composition of Alkyd 73-4-DEN 3 2 8 vs. maximum strength
C
1
I
I
1 E X T E N T OF
CURE
Figure 5. Theoretical relationship between extent of cure and maximum stress VOL. 2
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JUNE 1 9 6 3
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1
~
20
I
I
40
60
CURE
TIME
I
I
80 I00 (HOURS)
J
1 I20
Figure 6. Maximum stress vs. cure time and temperature for Systems A and B
---I
I
compare different polymers under equivalent circumstances they should be compared a t the same reference region. T o determine these regions for System A, a strength us. cure profile was run a t 140' C. (Figure 6). The lap-shear bond strength reaches a plateau a t a curing time of approximately 20 hours. This plateau is rather broad, with no indication of loss in strength until 65 hours of cure was exceeded. System B, also shown in Figure 6, was examined a t 140°, 160°, and 200' C. a t optimum compositions of 1.2/1, l . l / l , and 0.8/1 Alkyd 73-4-DEN 438, respectively. At 140' C., the lap-shear strength appeared to reach a plateau in about 24 hours a t about 4600 p.s.i. However, there was a gradual increase in strength to 5100 p.s.i. a t 120 hours' cure. A plateau a t 5200 p.s.i. was reached in about 40 hours a t 160' C. cure and there was no evidence of degradation even after 120 hours. T h e curing temperature of 200' C. reached a peak in about 18 hours; however, degradation was noted in a relatively short time. These regions are used as reference points in the remaining part of this study. Effect of Joint Thickness. A quick study was aIso made on the effect of bond thickness on joint strength. McBain (75) established that the strength of a n adhered joint increased with decreasing thickness of the glue line. This seems to be true of all rigid adhesives. Three types of explanation have been offered for this behavior, and one of them assumes the existence of long-range surface forces. The second suggestion involves the theory of flaws. As the glue line becomes thicker, 88
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
the probability of flaws becomes greater. A third explanation involves "built-in" stresses and has not yet been formulated quantitatively. The results for System A for thin glue lines, shown in Figure 7, are in general agreement Lvith the proposed theories; however, below 0.003 inch a starved joint was obtained. This same behavior was experienced by Koehn (74) in testing adhesives in tension and shear. There is. therefore, a minimum glue line thickness necessary to mate the adherends, which is a function of surface roughness and rigidity. O n the basis of the results of this experiment a glue line thickness of 0.004 inch was used in the lap-shear tests. Torsion Pendulum. LOCATIONOF GLASS TRASSITIOS TEMPERATURE. The modulus and damping curves over a temperature range tell a great deal about a given material. For a homogeneous system, a damping peak occurs in the softening range of the plastic. Sharp damping peaks indicate a narrow transition region. T h e ASTM heat distortion temperature occurs where the modulus curve starts to drop off and the damping curve begins to increase toward its peak. The glass transition temperature. Tg,is generally found at about the same temperature. Many systems exhibit multiple dispersion regions in the glassy state (7). These secondary dispersions have been attributed to the coexistence of crystalline and amorphous regions, to movements of smaller chain segments, and to the coexistence of separate chemical phases because of chemical heterogeneity or incompatibility. Referring to Figures 5 and 6, torsion bars for System A4\cere cured a t different curing times to examine Region B, the viscosity transition region (9-hour cure), Region C, the molecular independent region (24-hour cure), and Region D, the degradation region (65-hour cure). Region ,4 did not exhibit sufficient strength to test as a n adhesive. Figure 8 shows the dynamic behavior of the 24hour-cured specimen. The absence of multiple dispersion regions of the damping curve indicated polymer homogeneity. A maximum in the damping us. temperature curves could not be obtained because of excessive damping above the Tg region. T h e damping peak is approximately 15' to 20' C. above the commonly observed glass transition temperature. The glass transition temperature, Tg,is not a well defined point but rather a region which depends on the type of experiment used to locate it, the rate at which the experiment is performed. and to some extent the judgment of the experimenter. Most of the experimental work involving Tg and mechanical properties has been insensitized by examining a particular material a t a considerable distance from Tg,such 50" C. This is done because Tg is difficult to locate as Tg precisely and mechanical properties change drastically at this point, so that a small miscalculation could lead to erroneous results. In this study, however, the Tg region is important and its location was attempted within a range of + 5' by extrapolating the linear sections of the loss curves to a point of intersection, which was considered to be Tg 5" C. EFFECTSOF STRUCTURE.The 24-hour curve of System -4 (Figure 8) shows a Tg of 5" f 5" C. The damping and modulus curves are similar in shape to those shown by Nielsen and Buchdahl ( 78) for cross-linked phenol-formaldehyde. They shoic. the difference in the form of a slightly cross-linked and highly cross-linked resin. Figure 8 falls somewhere in between these extremes, as \could be expected if the crosslinking density was compared to that of a highly cross-linked density. May and Weir (76) studied the low shear dynamic properties of a commercial diglycidyl ether of bisphenol -4 cured with m-phenylenediamine, which is considered a typical, well-cured epoxy resin system. Interpreting their
+
*
'\
\\ \
\ \
\
!-
0.6
0.5
0.5 -
!-
I-
0.4
f,
z W
2n
0.4-
1
w
a
a
0.3
0.3n
LI
5 a2 n z
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ma
n
0. I
0 -1
0.2-
9n
U
1
-30
1
-20
1
I
-10 0 TEMPERATURE rc)
IO
L i
Figure 8. Torsion modulus and damping decrement vs. temperature for System A cured for 24 hours
curves in the same manner as in this study, a Tg of 130' i 5" C. was obtained. Using this system as a reference, it can be seen that the effect of copolymerizing with Alkyd 73-4 instead of curing with m-phenylenediamine has reduced Tg by approximately 125' C. T h e reduction is in accordance with DiMarzio (70), who found that the stiffness energy of X-Y (a copolymer of monomers X and Y) bears no unique relation to the stiffness energies of X-X and Y-Y links of the homopolymers. Although System A cannot be directly compared on a mole ratio basis with its homopolymers, the dominating effect of the lower melting point component, Alkyd 73-4, is shown indirectly by its use with other epoxy resins. EFFECTOF CURING CONDITIONS. T h e effect of cure time on System A is shown in Figure 3. T h e 9-hour cure shows damping properties very similar to the 24-hour cure; however, the shear modulus is lower in the glassy region. There is a progressive, although very slight. increase in Tg,as the curing time is increased to 45 hours. .4t 65 hours, however, where the strength us. cure time profile showed the beginning of strength degradation, the specimen was more temperaturesensitive, showing a loss in shear strength, a sharper damping peak, and a Tg of -3" ==I 5' C. This degrading may have been caused by chain scission or. possibly, in reference to the fracture theory, disappearance of lower molecular weight material. Nielsen (77) showed that chemical heterogeneity will broaden the transition region \vithout changing the position of the maximum. Therefore, as curing is extended, it is conceivable that the material of lower molecular weight disappears, leading to a less heterogeneous system and a sharper transition region. T h e replacement of Epon 828 with D E N 438 (System B) increased the Tg of the resin systems to 25' =t5' C. (see Figure
0
Figure
9.
Torsion modulus and damping decrement for System A
vs. temperature
10) for a resin cured to the threshold of molecular weight independence (41 hours). .4n increase in Tg of about 40" C. is realized for typically amine-cured epoxy novolacs (71). However, copolymerizing with Alkyd 73-4 appears to have a dominating effect on Tg. Both systems also exhibit very similar damping and modulus curves, other than a shift of about 20" C. u p the temperature scale for System B. In comparing the effect of cure temperature on Tg a t the same extent of cure there is approximately a 5' C. shift in Tg as the cure temperature is raised from 140" to 160' C. (Figure 6), and about the same shift from 160" to 200" C. (Figure 11). Figure 12 shoivs that the effect of curing temperature and time can be folloived Jvith torsion data. At 200" C. for 72 hours, which according to Figure 6 has undergone a degradation of lap-shear bond strength, Tg has increased to 35' f 5' C. T h e apparently higher cross-linked material exhibits a behavior similar to that observed by Nielsen and Buchdahl (18) for highly cross-linked phenol-formaldehyde. T h e effects of cure time and temperature are summarized in Table I.
Table 1.
Effects of Curing Conditions on Glass Transition Temperature Ciird - .. .
System .A
B
Cure Times, Hours 9, 24, 45 65 24 120 4 41 120 18 72
VOL. 2
Tern$.,
c.
140 140 140 140 160 160 160 200 200
NO. 2
Tg,
C.
5zk5
- 3 zk 5 18 i 5 25 =t5 18 5 25 i 5 30 3= 5 30 zk 5 35 f 5
+
JUNE 1 9 6 3
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! 0.5 O ' T L
5
a5
i
I
0.4
c
5 0.4
I
t I,
f
a
2
0.3
a
7j
f
w
0
i
! P
0.3
(1
f
0.2
a
I-' 1
-60 T E M P E R A T U R E (*C)
Figure 10. Torsion modulus and damping decrement vs. temperature for System B cured a t 160" C. for 41 hours
CURE TEMP ('C) 0
140
0
160
200
2
0.2
a 0.I
4 0
-40
-do
0
do
d,
20
TEMPERATURE ( C )
Figure 1 1 . Effect of cure temperature on torsion modulus and damping decrement of System B 90
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
'
J
0.6 r
-40
20 40 T E M P E R A T U R E (*C)
-20
0
60
J
Figure 12. Effect of cure time on torsion modulus and damping decrement of System B cured a t 200' C.
I t is obvious that once the molecular weight independence region is reached, cure time and temperature have very little effect on moving Tg. For some unexplained reason, the damping capacity of the 72-hour cure a t the lower temperatures was higher than the 18-hour cure. In a n attempt to rerun this experiment, the several points checked fell slightly below the 18-hour values. Polymeric materials that are repeatedly stressed undergo molecular rearrangement. ,4lthough the torsion work could be duplicated with different test bars cast from the same batch of resin, repeated runs on the same bar showed effects of work hardening and fatigue (Figure 13). I t is believed that the longer cure a t 200" C. produced a structure so highly cross-linked as to resist the "freezing-in" effects of low temperature-i.e., the increase in packing density was resisted. Stressing the material apparently increased molecular order, which contributed to the stiffness of the poly-mer. The curve for 120 hours a t 140' C. coincides with the 41hour cure a t 160' C. in the region above a n ambient temperature of 10" C. Referring to Figure 6, it can be seen that the lap shear strength of the 120-hour cure a t 140' C. equals that of the 41-hour cure a t 160' C. Electrical Measurements. LOCATION OF GLASSTRANSITIOK TEMPERATCRE. Locating the viscoelastic regions of the resin systems with electrical measurements makes it possible to investigate the resin in the bonded state, which may be considerably different from the bulk polymer. The electrical measurements were not limited in temperature range as were the torsion experiments. The accuracy of the electrical measurements exceeded that necessary to determine Tg, since only relative values were needed. The accuracy of the
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-40
-60
Figure 13.
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- 20
-40
-
a
=a 8 -
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4 )
0
5 IO
0
/ 1
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4-
I
01
I
40
I kc
o 0
"2 x
2,
I
30
FREQUENCY
10-
% I!
I
20
Fatigue effect of repeated torsion tests on System B
I
E
I
FREQUENCY
-60
E
I
-10 0 IO TEMPERATURE ( C )
-20
0
0
:?
,a I
8
0
o
u
I
.. I
I
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3
Figure 15. Effect of frequency on loss factor vs. temperature of System B cured at 160" C. for 41 hours VOL. 2
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91
experiments depended largely on the thickness measurements. An error in thickness has little or no effect in locating Tg, but it does affect the absolute values and the height of the damping peak. A 100% error in a thickness of 0.002 inch gives no apparent change in position of Tg, only a difference in the sharpness of the peak. Reruns on the same samples yielded a n almost exact duplicate of the original curve. Reproducibility between samples was within experimental error when thickness was accurately known. FREQUENCY EFFECTS. Since frequency was expected to affect Tg (6, 23), preliminary data were obtained on ten frequencies ranging from 100 C.P.S. to 100 kc. There were no indications of any gross effects on Tg. Figures 14 and 1 5 show the apparent lack of influence for 1. 5 . and 10 kc. on Systems A and B. O n the basis of this information, a frequency of 1 kc. was arbitrarily chosen to determine Tg. THICKNESS EFFECT. Choosing the optimum curing condition of 24 hours a t 140' C., System A was investigated a t various joint thicknesses. As can be seen in Figure 16. thicknesses ranging from 0.002 to 0.008 inch had no noticeable effect on Tg. I t was felt that if the ratio of surface area to volume of the bonded resin was increased. the effects of restricted molecular mobility at the interface would raise the effective Tg. A more refined system with possibly a larger specimen would be necessary to explore this subject more precisely. A secondary peak was obtained in the region of 65' to 70' C. for all thickness. This peak could be caused by the "pinch effect" described by T y (25). Assuming that the adhesive forces are strong enough, when the bonded specimen is heated, the greater thermal expansion of the aluminum would tend to stress the polymers adsorbed on the surface. The restriction of molecular mobility could produce the peak. T h e sharp increase in the region of 90' C. would then be the temperature a t which the interfacial restriction is overcome by thermal energy or relaxation of the polymer. Without even considering differences in thermal expansion, the fact that adhesive forces would restrict mobility in the vicinity of the interface. as opposed to the bulk of the resin, could account for the peak. The apparent flattening of the peak with decreasing thickness could indicate that the greater percentage of surface-bound molecules resist relaxation up to the point where the adhesive forces relax, thus eliminating the peak. Another explanation supposes development of phase heterogeneity due to one or more of the three effects mentioned previously. However. looking a t a very thick joint of 0.019 inch of System B (Figure 17). it is seen that the peak has disappeared. with only a change in slope in the temperature region. This strongly suggests that the surface effects are masked by the greater volume of bulk material. A more elaborate torsion pendulum might settle this issue. Figure 17 also shows that it is possible to follow curing conditions with electrical studies. C o w ~ R I s o xOF TORSION AND ELECTRICAL METHODS.T h e Tg of the two methods is compared in Table 11.
Table II.
System A B
B
92
20
I
1 i
i
'
O
!
I
L I
01
too
0 50 TEMPERATURE ('C)
-50
Figure 16. Effect of thickness on loss factor vs. temperature of System A
48hr-4mils
;I
60
Glass-Transition Values Obtained by Torsion Pendulum Data and Dielectric Loss Factors Cure Time, Hours
Cure Temp.,
24 41 120
140 160 1GO
c.
Tg,
Torsion 5 f 5 25 f 5 30 i 5
C. Electrical 35 i 5 44 f 5 46 i 5
l&EC PRODUCT RESEARCH A N D DEVELOPMENT
o
'
A
"
l
'
50l
'
"
l
'
l
10l0
'
'
I50
TEMPERATURE ('C) Figure 17. Loss factor vs. temperature of System B a t 48- and 124-hour cures and various thicknesses
Tg obtained by dielectric measurements was about 20" C. higher than the low shear dynamic measurements a t about 1 C.P.S. This effect apparently is not a thickness factor, as shown in Figures 16 and 17; therefore it must be a frequency effect. Although very little difference in frequency was found between 100 C.P.S.and 100 kc., there was probably a shift in the lower regions between 1 and 100 C.P.S. T h e increase of Tg with frequency is due to the relationship of the relaxation times of the molecular segments to the frequency of the driving force. .4s the measuring frequency is increased, the relaxation time of the molecular segments must become shorter in order to maintain their phase position with the driving force. Therefore. a corresponding loss factor would be found a t a higher temperature as frequency is increased. This effect is \vel1 established ( 6 ) and its magnitude has been shown to depend on the type of system involved (79). EFFECTSOF STRUCTURAL VARIATIOSS. May and IVeir (76) studied the effect of substituents on the central carbon bridge of the diglycidyl ether of bisphanol .4 cured v i t h mphenylenediamine. By replacing one of the methll groups with a phenyl group, Tg was increased b)- 10' C. This increase was attributed to the restriction of rotation around the bridge. The epoxy intermediate, Epon 828 of System A, was replaced \\ith this phenyl-substituted material on an equal molar basis. This new epoxy intermediate has the follo\ving structure :
00
1
701
A
0
50 TEMPERATURE (*C)
I?
100
Figure 18. Comparison of loss factor vs. temperature between Systems A and C
and is the diglycidyl ether of 2,2-bis(p-hydroxyplie~iyl)-2phenylethane? supplied by the Shell Development Co., Emeryville, Calif. T h e effect of the phenyl group substitution in the copolymer with Alkyd 73-4 (System C) is shown in Figure 18. T h e Tg of this system is about the same as that of System A. This does not come as a surprise, since the dominating effect of the alkyd component on determining Tg has been established. However: the secondary peak is much more pronounced than in System A. Since one of the main purposes of this investigation ?vas to explore a possible relationship between Tg and joint strength, a typical epoxy resin with a known Tg considerably higher than that of the systems studied was examined. The first system consisted of the following formulation:
tions. this system cured for 30 minutes a t 121' C.. then postcured for 6 hours a t 160" C., gave a Tg of 130' C . From the electrical studies on this resin (Figure 19) a Tg of 127' i 5' C. \vas obtained. The close agreement indicates a lack of frequency effect on this resin system. The lap-shear tests showed that failure was interfacial and that the cohesive strength was not a controlled factor. In an attempt to eliminate this condition, one of the manufacturer's recipes (27) was used: M o l e Ratio 1 .o 0.67 0.17
Epoxide 201 Maleic anhydride
Ethylene glycol
rvt. yo Epoxide 201 Maleic anhydride Trimethylolpropane
19.2
.Uole Ratio 1 .o 0.7
2.6
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Epoxide 201 ( E P 201) is the commercial name of a dicyclodiepoxycarboxylate monomer made by Union Carbide Chemi'.4 '-epoxycals Co., 3.4-epoxy-6-methylcyclohexylmethyl-3 6 '-niethylcyclohexanecarboxylate. Its structural formula is : 0
.4ccording to Patrick. McGary. and Phillips (27),this recipe has a heat distortion temperature or 168" C. As shown in 5' C . on bonded specimens \\-as Figure 19. a T,o of 145' obtained by dielectric studies. For cross-linked plastics, the heat distortion temperature would be expected to be ivithin roughly 15' to 20' of the glass transition temperature. O n this basis, a Tc of 145' C. appears to be reasonable. I n another similar system. the ethylene glycol was replaced u i t h the same number of moles of dipropylene glycol. T h e effect of this long. linear, flexible glycol molecule on T g is also found in Figure 19. T q \\-as reduced by this internal plasticization 5' C . from 145' to 55' Internal plascicization v-ith these low melting point diols behaves like the alkyd in Systems .-Z and B to produce a very critical effect on moving Tc;as compared to the slight shift caused by irxreasing the degree of cure.
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This polymer system as chosen because information was available on Tg and strength properties of lap-shear joints ( 3 ) . hccording to Barenholtz's specific volume determina-
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Loss factor vs. temperature for various recipes of Epoxide 201
Lap-Shear Strength. TEMPERATURE EFFECT. All the points shown for lap-shear data are based on a t least three, but mostly four, values. T h e average reproducibility of each Where possible, the data are set of points was within *lo%. pIotted in rectangles representing the spread of data. Where this method led to confusion, only the average values were plotted. Figure 20 shows the effect of temperature on the lap-shear strengths of Systems A, B, and C. System A shows a slight increase in strength from -60' to 0' C. (1500 to 2000 p.s.i.). From 0' to 20' C., there is a rapid increase to a peak value of approximately 2900 p.s.i. Beyond 20' C., there is a rapid loss in strength. System B also peaks in the vicinity of 20' C. However, the width of the optimum range is about double that of System A. The bond strength of System B is about 2000 p s i . greater than System A. Since cohesive strength is being measured, the difference is probably due to the higher cohesive properties of the more cross-linked system. The substitution of a phenyl group on the bridge (System C), which showed the same electrical data as System A also shows the same mechanical behavior. Apparently, the bulking value of the phenyl group offsets any noticeable increase in Tg due to restricted rotation around the bridge.
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I&EC PRODUCT RESEARCH A N D DEVELOPMENT
T h e results of the three Epoxide 201 systems are shown in Figure 21. T h e first system, which contained trimethylolpropane, gave very poor bond strength. Failure, as determined by a n optical microscope, was approximately 100% adhesional. Therefore, the relationship of resin to its viscoelastic properties was not evident. T h e strength values obtained were in the region of 2000 p.s.i., which is considerably experienced by Barenholtz ( 3 ) on primed steel. T h e substitution of ethylene glycol for trimethylolpropane still gave a system that produced from 50 to 75% adhesion failure upon breaking. However, even though the actual values are not strictly cohesional, a peak in strength was found a t about 130' C. By replacing ethylene glycol with dipropylene glycol, the width of the peak was broadened. The range of this optimum region extended from 55' C. (the Tg of the system measured by electrical means) to 135' C. (the Tg of the former system). RELATIONSHIP B E T h T E N Tg AND MAXIMUM BOSDSTRESGTH. I n all the systems examined, there appears to be a close relationship between Tg and ultimate bond strength. According to Smith and Magnusson (23), amorphous polymers are in corresponding states when the temperature differences (TTg)are equal. A plot of System ,4 and B in the form of stress us. ( T - Tg), \There Tg was determined by torsion data, is shoisn in Figure 22. The optimum region of System .4 is Tg 10' to Tg 25' C., whereas the optimum region of System B is Tg -25' to Tg 10'. I t appears that the more linear System .4with a sharper damping peak extends further beyond Tg to reach optimum properties, where System B exhibits ultimate properties below Tg. Figure 23 shoivs the same plot for all the resin systems; however, it is based on Tg determined by dielectric studies. The damping peak of the electrical studies, which would be expected to be sharper because measurements are being made on only molecular motions, is further in the Tg region (approximately Tg 20" C , ) , Therefore, the plots of Figure 23 are shifted to the low side of Tg. If there is a relationship between the viscoelastic properties of an adhesive and joint strength, there should also be one with rate of rupture ( 8 ) . Increasing the rate of separation should have the same effect as lowering the temperature, since the slorver moving mechanisms are frozen out by time. Figure 24 sho\vs this effect on System A. At a separation rate of 0.02 inch per minute, the ultimate bond strength peaks around 15' C. \Vhen the rate was changed to 0.20 and 20.0 inches per minute, the peak increased progressively from 25' to 40' C., respectively. There is roughly a 10' C. increase for each 10-fold increase in rate. Smith and Magnusson (23) have shown a similar relationship between the tensile strength of various elastomers and Tg a t values above Tg. Lap-shear relationships with temperature for various types of epoxy adhesives are shown by Gould (73). H e found a shift in the optimum range when using what is known as an aircraft hardener, which apparently raises Tg of the system. POSSIBLE ROLEO F FRACTURE THEORY. T h e existing theories on viscoelastic behavior of amorphous polymers do not adequately explain ultimate properties. Because of the many complexities occurring a t failure, a fracture theory may be more applicable. T h e relationship between Tg and optimum mechanical strength would be expected if the crack propagation theory is considered. T h e actual strengths of materials are orders of magnitudes less than their theoretical strengths. This is attributed to existing flaws in the material. T h e ability to concentrate energy in their vicinity and to cause flow into a
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full-fledged crack has been ascribed to these flaws (72, 22). Brittle materials can be shown to fail almost as soon as a crack starts propagating. For ductile materials capable of considerable plastic deformation, the presence of holes, notches, and other flaws does not result in stress concentrations leading to fracture, because any stress concentration a t a particular point in the material is eliminated by plastic deformation, which causes a more uniform stress distribution. According to Orowan (20),the stress required to propagate a crack in metals is given by the formula :
where u = tensile stress required to propagate a crack of length c y = surface energy of the fractured faces E = Young's modulus p = plastic strain energy factor Although Equation 3 may not be quantitatively correct, the qualitative principles have been observed to hold for many amorphous polymers. Above the transition temperature p is large; therefore u will also be large, providing the increase in p is much greater than the loss in E. Below the Tg region, the material is brittle a n d p is low; therefore the stress necessary to cause crack growth will be reduced. LVith this approach, a n ideal structural adhesive would be one with a high E value to give strength and rigidity and a high loss factor, 6, to prevent crack propagation. These two properties are generally not coincident in a given state of a polymer. I n the glassy region, below Tg, E is high but 6 is low. which favors brittle failure. Above Tg.E is low and 6 is high, giving little cohesive strength. The threshold of the Tg region, however, where the dissipation factor increases rapidly, while E remains high, would appear to be the most favorable state for ultimate bond strength The crack propagation approach in these systems has been qualitatively supported in several ways. Figure 25 shows the effect of cure temperature on strength. At a 200" C. cure temperature, the region of optimum strength narrows. A very plausible explanation for this behavior could be the absence of low molecular weight material after the more severe curing conditions. The lower molecular weight component acts as a crack healing agent. I t could be considered then that the system is a heterogeneous one due to molecular weight distribution, but as curing is increased it becomes more homogeneous. This may also explain the effect of using dipropylene glycol in the Epoxide 201 system. The plasticization effect of dipropylene glycol lowered the Tg of the system; but the region of optimum strength was in-
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creased considerably. Bobalek and Evans (5) found that a similar approach was necessary to promote good impact resistance over a broad temperature range. The unusual toughness below Tg that is exhibited by certain epoxy resins is attributed here to the flexibilizing glycol portion, which according to May and Weir (76) gives a damping peak a t -60" C. and would conceivably retain a considerable p value of Equation 3. The high temperature strength is attributed to the rigid phenyl portion of the molecule. Because of the time-dependent factor involved in stressing a plastic, it is expected that as the rate of stressing is increased the ultimate strength will increase and the energy to rupture will decrease. If the plastics undergo a large reduction in flow properties a t the higher rates, as seen by the reduction of p in Equation 3, ultimate strength may actually decrease. Figure 24 shows a slight reduction in ultimate strength properties a t faster rates. D I F F E R E N T I A L S H R I S K A G E EFFECTS.An attempt was made to obtain some information on the effect of stresses due to differential shrinkage of Systems A, B: and C us. the aluminum adherends. A set of four samples of each was packed in dry ice, cooled to - 80" C., allowed to warm up to room temperature, tested, and compared with those held a t room temperature. In all three cases there was no significant difference in bond strength due to cooling. I t was concluded, therefore, that loss in bond strength below Tg was not a stress factor caused by differential thermal shrinkage. Conclusions
The viscoelastic properties of an adhesive must be known in order to make a n intelligent and systematic comparison between different adhesive systems. Maximum bond strength of lap shear specimens of the systems studied peaked near the threshold of Tg or in the region of Tg ?Z 20' C. depending on the nature of the adhesive system. The width of this peak strength depends on the local molecular architecture in the adhesive polymer. To get tensile strength along with toughness, a certain degree of heterogeneity is necessary, which will provide a proper balance of elastic and plastic properties. Whether it is best that this heterogeneity is built into the chain backbone. a factor of molecular weight distribution, or a polyblend of rigid and rubbery materials is not yet known. All three are being experimented with by industry. The above observations are consistent with fracture theory, These relationships are valid only if the cohesive strength of the adhesive is the controlling strength factor-that is, if the adhesion forces a t the interface are greater than the cohesive strength of the adhesive. Valuable information can be obtained on the viscoelastic behavior of the bulk properties of adhesives using torsion tests, and also on bonded properties of adhesives using dielectric measurements. Using either method. the region of optimum strength properties can be located and it is believed that, with more intensive studies in this area, the width of these regions and the molecular architectural arrangements responsible for this phenomenon can be predicted. There is very close agreement between Tg determined by torsion tests and ultimate joint strength. The apparent Tg measured by electrical methods is approximately 20' C. higher than by the former method. Both methods are capable of following the degree of cure of an adhesive. Once an amorphous, cross-linked polymeric adhesive system has reached the threshold of molecular weight independence. further curing does nor move the Tg significantly. Driving the polymerization further has a degrading effect on bond per-
formance. because the temperature range of peak performance is narroLved. T h e Tg can be moved about very drastically by proper selection of curing agents or copolymers. T h e Tg of rigid adhesive systems can be reduced by introducing flexible components into the system. Besides the relationship of viscoelastic properties and joint strength, a new adhesive system has been investigated. This system can be cured without the use of catalysts or solvents and gives good adhesive bonding to aluminum for the temperature region of -15’ to + 4 5 O C. However, this study is by no means a complete analysis of a n adhesive system. Many other factors. such as long-time loading effect, water and chemical resistance. and exposure to various environmental conditions are necessary to determine whether or not an adhesive will perform a specific task.
Birhdahl. R.., Nielsm. L. E.. J . Abb1. Phvs. 21. 482 (19501 uchdahl; E., Kielsen,’L. E.,‘J.Po’bmer &. 151 1 (1955). ’ 8) B--Burrell, H., Ofic. Die. Fed. Soc. Paint Technol. 34, 131 (1962). (9) Chiang, N.-T., “Preparation and Properties of Alkyds with More Complex Carboxyl Functionality,” Ph.D. thesis, Case Institute of Technology, 1963. (10) DiMarzio, E. A., Gibbs, J. H., J . Polymer Sci. 40, 12 (1959). (11) Dow Chemical Co., Dow Epoxy Novalac Booklet. (12) Frankel, J. P., “Principles of the Properties of Mate1 ials,” p. 180, McGraw-Hill, New York, 1957. (13) Gould, Bernard, Prod. Eng. 28, 64 (1957). (14) Koehn, G. W., “Adhesion and Adhesives, Fundamentals and Practice,” p. 120, Wiley, New York, 1954. (15) McBain, J. W., 2nd Report, Adhesive Research Committee, H.M.S.O., London, 1926; 3rd Report, H.M.S.O., London,
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(16) May, C. A , , IVeir, F. E., S P E Tran. (July 1962). (17) Nielsen, L. E., J . Am. Chem. Soc. 75, 1435 (1953). (18) Nielsen, L. E., Buchdahl, R., S P E J . 9, 16 (May 1953). (19) Nielsen, L. E., Buchdahl, R., Levreault, R., J . Appl. Phys. .. 21, 607 (1950). (20) Orowan. E.. “Fatigue and Fracture of Metals,” P. 154, ’ &ley, New York, 1955. (21) Patrick, C. T., McGary, C. W., Phillips, B., “Unox, Epoxide 201 Booklet,” Union Carbide Chemicals Co., New York. (22) Peckner, D., Mater. Design. Eng. 51, 127 (.4pril 1960). (23) Smith, T. L., Magnusson, A. B., J . Polymer Sci. 42, 391 (1960). (24) Tobalsky, A. V., “Properties and Structure of Polymers,” p. 71, Wiley, New York, 1960. (25) Ty, L. N., “Structural Variables .4ffecting Electrical Properties in Epoxy Adhesives on Steel,” Ph.D. thesis, Case Institute of Technology, 1961. (26) Ward, R. J., “Correlations between Molecular Structure and Some Thermal, Electrical and Mechanical Properties of .4lkydEDOXV CoDolvmer Adhesives.” Ph.D. thesis, State University C’ollege of Forestry, Syracuse,“. Y., 1962. ~
Acknowledgment
Recognition is due to the Wright Air Development Division, \\’right-Patterson ,4ir Force Base, Ohio, for partial support of this work under contract No. ,4F33(616)-7210. Literature Cited (1) Am. Sac. Testing Materials, Philadelphia, ASTM Standards, n liO.54T ( 2 ) Zbzd., D 1002-53T. ( 3 ) Barenholtz, George, “Correlation of Some Thermal Properties of EDOXV Adhesives and Shear Streneth of Steel Laminates,” rnast‘er’s’thesis, Case Institute of Techndogy: 1960. (4) Bickerman, J. J., “The Science of Adhesive Joints,” p. 125, Academic Press, New York, 1961. (5) Bobalek, E. G., Evans, R. M., S P E Tram. 1, 93 (1961). I
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RECEIVED for review October 29, 1962 ACCEPTED March 13, 1963
QUATERNARY ANTISTATIC AGENTS EFFECTIVE IN PLASTICS ALLAN E. S H E R R A N D EMIL A . V I T A L I S American Cyanamid C h . , Stamford, Conn.
Antistatic agents for surface treatment of materials are well established; however, incorporation of such agents into plastics has been difficult. The development of cationic quaternary compounds suitable for use in poly(viny1chloride), polyethylene, polypropylene, polystyrene, and related polymers i s reviewed. Plastics containing the agents are shown to retain antistatic activity for long periods. Syntheses, chemical and physical properties, heat stability, diffusion data, and interactions with other additives are described. Methods of incorporaring the antistatic agent are suggested.
suitable for surface treatment of materials have been well established, but difficulties were encountered when attempts were made to incorporate these agents into plastics. The development of certain cationic quaternary compounds suitable for incorporation into plastics is revie\ved here. Research to determine what types of organic structures are effective for imparting nonstatic properties to textiles has been in progress a t American Cyanamid for over 12 years. As a result of these efforts, it was learned that quaternary ammonium compounds in general, and particularly structures of the Catanac SN antistatic agent type (American Cyanamid
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