SUPPORT POLE PIECE
TO VACUUM PUMP VACUUM “0’ RING SEAL
SUPPORT COIL LIGHT SOUR SCANNING BRIGHT METAL PART OF ROTOR
SCANNING POINT SPOT
/ I l pz
AUTO MAT IC CONTROL FOR SUPPORTING CIRCUIT
c
r
DAMPING NEEDLE 0 COIL
PHOTO TUBE’
The adhesion tester is an electromagnetically supported, electrically driven centrifuge
I
HARVEY ALTER’ and WALTER SOLLER Applied Science Research Laboratory, University of Cincinnati, Cincinnati 2 1, Ohio
Molecular Structure as a Basis for Adhesion Ultracentrifugal Measurement of the Adhesion of Epoxy Polymer An important new tool has been used for measuring adhesion uncomplicated by related factors
THE disagreement between theory and practice in discussing the adhesion of polymers is well known to workers in the field. One reason has been inability to obtain quantitative adhesion measurements. To date, only one known instru-
Figure 1.
Ultracentrifuge rotors
Material.
922
ment is capable of measuring the force of adhesion itself for an amorphous coating: the University of Cincinnati ultracentrifugal adhesion tester, which was used in the study reported.
4340 steel
INDUSTRIAL AND ENGINEERING CHEMISTRY
’
?he Ultracentrifuge
The ultracentrifugal adhesion tester is an electromagnetically supported, electrically driven centrifuge, capable of speeds up to 43,500 revolutions per second (73). The design of the rotors is shown in Figure 1. The present model employs a two-phase drive system and differs greatly from previous models in that control of the vertical position of the rotor is automatic for many hours. The rotors are extremely stable over long periods of time and fewer than 1% are lost through erratic behavior. Changes in circuitry and mechanical design ensure more stable operation and more positive coupling between the rotor and the damping needle. 1
Present address, Development Depart-
ment, Bakelite Co., Division Union Carbide
Corp., Bound Brook, N. J.
SO0
W v)
2400 Pcz 0 W
c
$300
0 n
>
t In
8 20c
2 sY lb
3 100 0 I
20
60
I
100
I
I
140
180
m a
TIME, MINUTES
+
Figure 2. Viscosity increases with time for Epon 1001 cured with 4 p.p.h. diethylenetriamine
Materials
60
100
140
180
TIME, MINUTES
Epon 100 1, 57% solids with equivalent amount
Figure 3. of amine
A spot of polymer is applied to the cylindrical surface of the rotor and treated as prescribed. The rotor is then placed in the glass tube and the system is evacuated to a pressure of 0.1 to 0.4 micron of mercury; the rotor is supported and rotated. A beam of light is reflected from the rotor to a phototube which is electrically connected to an oscilloscope. The oscilloscope scanning frequency is kept matched to the slowly increasing speed of the rotor, so that the output from the phototube controlling the deflection of the electron beam of the oscilloscope gives a fixed pattern on the screen. The scanning frequency of the oscilloscope is electrically connected to a frequency meter. When the polymer spot is thrown off the rotor, there is an abrupt change in the oscilloscope pattern. The scanning frequency, which is equal to the speed of the rotor, is read from the meter and this is the angular speed of the rotor a t failure. The optical-electrical detecting system is sensitive enough to detect a transparent polymer spot on a highly polished steel rotor. The failure stress is calculated (73) by knowing the sample thickness, density, and the speed necessary to remove it. The mean of from six to twelve measurements is reported. The polymer spot is applied to the rotor using the apparatus described (73), but slightly modified to give greater control over spot diameter, thickness, shape, and position on the rotor. Instead of a small bowl-shaped brass cup, a small concave tip is installed a t the end of the lever arm. The size of the brass tip is matched with the viscosity and drying characteristics of the polymer solution, so as to control spot geometry. The polymer solution is applied at room temperature, preferably in a dust-free atmosphere.
20
and Techniques
Epoxy Polymer System. The polymer employed was Shell's Epon 1001, the condensation product of bisphenol A and epichlorohydrin. The samples were from three lots (A, B, and C), and the weight per epoxide (WPE, grams of resin containing 1 gram equivalent of epoxide) was supplied by the manufacturer for each lot. The polymer was applied from a solution of known concentration after addition of a previously determined amount of amine curing agent. The polymer was dissolved in a mixture of solvents consisting of 32% xylene, 32% methyl isobutyl ketone, 32% Cellosolve, and 4% cyclohexanol (by weight). The curing agent was added as a 50% by weight solution in thinner. The solvents used to prepare the thinner were the best grades available; they were all dried, distilled, and percolated through a
150 1=
column of alumina prior to use, to eliminate water, color bodies, and surface active impurities. The solvent purification was necessary to ensure blemishand defect-free films and consistent adhesion tests. The polyfunctional amines employed as curing agents were either freshly distilled or crystallized prior to use. Usual laboratory procedure was to add a calculated amount of amine to the polymer solution and follow the timedependent viscosity change with a Brookfield Model LVT viscometer at 30 r.p.m., a t 30' =I= 0.2" C. Typical plots of viscosity us. time, measured without solvent evaporation, for various solutions of Epon 1001 cured with 4 parts per hundred of diethylenetriamine are shown in Figure 2. The solutions represented are for two lots of material, differing in their epoxide equivalent values. Typical curves of viscosity us. time for solutions of Epon 1001 plus equivalent
1400
Figure 4. Adhesion as u function of bake time Epon 1001 formulation 4 p.p.h. DTA
I&
$
$1000
a 900 800
6 0 0 1 TIME OF BAKE 5 hourn at 50. C. + Indloaled time at 100. C.
VOL. 50, NO. 6
JUNE 1958
923
amounts of polyfunctional amines are shown in Figure 3. By properly choosing the concentration of the polymer in solution, the viscosity range covered can be chosen beforehand. Samples of polymer solution were withdrawn at intervals and placed in the coating apparatus, and replicate rotors coated. I n general, a 3- to 6-minute interval was required to coat 10 to 15 replicate rotors. The viscosity at coating was taken to be the average viscosity over this time interval. The viscosity of the Epon solutions is a function of both solids content (polymer concentration) and extent of reaction between the active hydrogen of the curing agent and the oxirane ring to form the amine alcohol grouping. The reaction has been discussed in detail by Shechter, Wynstra, and Kurkjy (76). Solvent Removal a n d Curing of Polymer Films. Long bake schedules are necessary before samples of cured Epon 1001 will give reliable, precise adhesion results with the ultracentrifuge. The ultracentrifuge apparently serves as a very sensitive instrument to detect in complete solvent removal and/or unreacted curing agent. Incomplete solvent removal is indicated by cohesive rather than adhesive failure of samples, the retained solvent apparently acting as a plasticizer giving soft films; incomplete cure is shown by low and nonconsistent results. The bake schedules recommended in the coatings technical literature neither completely removed the solvent nor allowed the cross-linking agent to react completely. Experimental evidence indicates that if the curing agent is not given a sufficient time or opportunityLe., high enough temperature-to react, maximum adhesion will not be obtained. For example, films permitted to stand for 8, 15, 23, and 30 days at room temperature before testing with the ultracentrifuge gave adhesive failures, but the force required to remove the films was approximately one third to one half that required for similar baked films. No significant difference in adhesion was observed for three samples-room temperature cure at the end of 30 days was no better in promoting adhesion than a t the end of 8 days. Figure 4 shons the mean adhesion of an Epon 1001 formulation cured with 4 p.p.h, of diethylenetriamine as a function of bake time. The samples were air-dried for 16 hours and baked 5 hours a t 50" C. and the indicated time at 100" C. The extent of cure was estimated by extraction of cured samples with methyl ethyl ketone. The cure schedules chosen gave samples more than 90% insoluble in the ketone. Recently reported work of Shechter, Wynstra, and Kurkjy (76),Dannenberg and Harp (5),and de Bruyne (7) indicates that the curing schedules should be more than sufficient to achieve "complete" cure and to an-
924
neal the films (6). As a consequence, the polymer samples tested in the ultracentrifuge were air-dried for 16 hours, followed by 5 hours' bake a t 50" C. and 24 or more hours at 100" C. The dry film density was determined a t room temperature (73). I t was established that 8 to 10 hours' bake is sufficient to remove the solvent-i.e., constant film density. The density was independent of the viscosity at coating, polymer concentration in the solution, differences in manufacturer's lot, curing agent structure, and concentration. A value of 1.177 grams per cc. was used in calculating the adhesive stress. Substrate Material a n d Effect of Surface Condition. The rotors for the ultracentrifuge were fabricated from 4340 steel and heat-treated by oil quench at 75" C. and tempered at 375' C. to yield a hardness of 50-52 Rockwell C. After this treatment, the steel has a tensile strength of approximately 300,000 p.s.i. at room temperature (75); high tensile strength is a prerequisite for the high speeds the rotor must endure. After heat treatment the rotors were polished between centers on a specially constructed lathe consecutively with 4/0 metallographic emery paper and levigated alumina. The alumina was ap. plied dry and in aqueous slurry backed with Fisher's Gama1 polishing cloth. After polishing, each rotor was checked under a wide-field microscope in a n attempt to ensure uniform surfaces. Three rotors withdrawn a t random from a group of 60 polished rotors showed a very low surface roughness of 2.8 r.m.s. microinches when measured with a Profilometer, varying from 2.3 to 3.3 r.rn.s. microinches for measurements at various points on the surface and across the direction of polishing. Polished rotors were stored under a dry hydrocarbon solvent prior to use. Before coating, the rotors were polished with dry alumina, washed with an organic emulsion cleaner, rinsed well with dry, chromatographed acetone, placed in a jig, blown dry with filtered compressed air, and coated. Removing from acetone to coating takes 15 to 20 seconds per rotor. The cleaning procedure gives an advancing contact angle of water on the steel of less than 9". A clean surface is very important (2). The centrifuge rotors were coated with an absolute minimum exposure to air. Prolonged exposure, even for as little as 3 to 5 minutes in a desiccator, contaminates the surface; generally poor precision and low adhesive stress values are obtained. The cleaning procedure was designed to minimize moisture in a n attempt to avoid rust and keep water away from the resin. The physical character of the surface is important. Some adhesion measurements of Epon 1001 and formulations of Epon 1001 were made on etched sur-
INDUSTRIAL AND ENGINEERING CHEMISTRY
faces and surfaces that were cleaned by blasting the steel rotor with levigated alumina in a stream of compressed air, followed by various solvent cleaning procedures. This type of treatment produced rough and pitted surfaces. Very few reliable data could be obtained by utilizing these surfaces and, in general, the results were widely scattered and showed anomalies. The situation was similar for acid-etched surfaces. Despite the arguments for increased surface area at the interface, it has been reported that a smooth surface gives a stronger joint than a rough one (74). O n a rough surface, the depressions must be filled in, incomplete wetting results (77), and air bubbles may be trapped. Such sites of interfacial discontinuity will cause high stress concentration and weaker joints (72, 75). This is about what was found when the adhesion of Epon 1001 was measured on a uniformly roughened surface produced by acid etching, compared with adhesion on a polished surface. The results summarized in Table I compare their group means and coefficients of variation.
Table 1.
Effect of Substrate Condition on Adhesion
Rotor Mean No. of Surface Failure, RepliCondition P.S.I. cates Acid etched 1225 ' 5 Polished 1342 7 Induced defects 1444 8 No induced defects 1602 10
Coefficient of Variation,
% 40.1 13.3 16.8
13.6
Table I also reports representative adhesion test results for samples with and without induced defects a t the interface. Rotors coated with Epon 1001 solution were identically treated, except that half, chosen at random, were dried and cured with the interface below the surface. Dissolved and entrapped gases will rise to the interface of half the samples and induce defects; the other half should be relatively free of interfacial defects. The effect of defects was similar to that of an etched surface; the samples with defects gave lower adhesion values and poorer precision. Similar results have been reported for acidetched surfaces when measuring the adhesion of Epon VI adhesive (7). Precision of Ultracentrifuge Measurement. The index of precision used by Malloy, Soller, and Roberts (73) to describe the adhesion measurement was the coefficient of variation, C.V., defined as C.V. = ( S / F ) X 100, for a series of measurements of mean F and variance S2. The coefficient of variation due to systematic errors in measurement was previously estimated as 2.5% (73). Inasmuch as the experimental coefficient of
ADVANCES IN ADHESIVES CONC. X W / W
3400
4
1400
3300
Figure 5. Relation of adhesion to viscosity
1300
. Epon 1001
c. g 1200 .2
3200
4 p.p.h. DTA
3100 3000
L
E Y
g
-: 2900
a
iioo
0
w e
2800
e
I000
W
z I
1400
9
900
(
1300 BROOKFIELD VISCOSITY
CPS. a1
30.C.
1200
variation varies from 4 to 14%, averag1100 ing about lo%, adhesion measurements 1000 were studied, to increase the precision of the measurement and find which factors Figure 6. Adhesion decreases sharply with viscosity 900 contribute to the error. T o take into account possible and known variables, a multiple regression % u n : with model was fitted and the significance of the results statistically tested. I n all, curing agent) and the extent of cure 126 observations were included in the when coated, and there is minimum adanalysis of seven independent variables. hesion at a viscosity of around 400 cps. Computation was facilitated by using the No concrete explanation can be ofIBM Type 650 magnetic drum data procfered for the minima; they may be reessing machine and Cohen's published lated to the observed thixotropy of the regression program (4). The independresin solutions. ent variables tested were the viscosity, The results for lot C (Figure 6) illussquare of the viscosity, equivalence ratio trate the effect of coating a t constant of amine to epoxide, WPE value of the concentration of polymer (60% by resin, thickness of the test sample, and weight) with changing chain ' length. major and minor axes of the test When the coatings were applied under sample (regarding it as an ellipse). The these conditions, diethylenetriamine was efficiency of the precision when the Iatadded to the polymer solution and perter three variables were omitted was mitted to react with no solvent evapora93%, and analysis of variance showed tion. In this way, polymer could be apthese variables to be nonsignificant at a plied at constant concentration and difhigh level. ferent viscosities, the viscosity increase The over-all error mean square was being due to the increasing chain length. 12,900 (p.s.i.)zand the coefficient of variUnder these conditions, the adhesion ation for the 126 observations tested was decreases sharply with viscosity. 10%. The 95% confidence limits on Conversely, when chain length is held the mean adhesion would then be f 2 0 constant and concentration increased, the p.s.i. (t test). adhesion increases with viscosity. Figure Experimental Results 7 illustrates the effect of coating with Epon 1001 cured with an equivalent Factors Affecting Adhesion. VISamount of 4,4'-(diamin0)diphenylmethCOSITY OF APPLIED SOLUTION.Figure 5 ane. The solid amine (melting point shows adhesion as a function of viscosity 94" C.) was added directly to a 65% solufor lots A and B of Epon 1001, both cured tion of Epon 1001 and by successive with 4 parts per hundred of diethylenetridilutions, coatings were applied at difamine. Figure 6 shows the same relation ferent viscosities. (Figure 3 illustrates for lots A and C, cured with an equivalent the low reactivity of this curing agent at amount of diethylenetriamine (one equivroom temperature.) This means that alent of amine to one equivalent of the chain length was constant over the epoxide) over a wider viscosity range. In intervals of coating and that the lowest Figures 5 and 6, and those following, each viscosity solution represents the least point represents the mean value of from concentrated solution. If there was any six to 12 measurements. I n certain cases, reaction at room temperature, the mathe adhesion is independent of the initial terial coated a t 224 cps. would have the concentration of polymer (weight per longest chain length. Figure 7 illuscent of nonvol5tiles before addition of
200 IBROOKFIELD
400 VISCOSITY
1000
800
600
CPS at 30' C.
trates the increase in adhesion with' an increase in concentration of the applied solution of a polymer with constant chain length. CURINGAGENTCONCENTRATION. Figure 8 illustrates the effect of diethylenetriamine concentration on the adhesion of lot B of the epoxy resin. The curves are described in terms of a parameter, r, defined as the ratio of equivalents of amine to equivalents of epoxide. In accord with Shechter's findings (76),no differentiation is made between the reaction of a primary or secondary amine group with the oxirane ring; the reactions of the oxirane
4000
3900 3800 3700
3600 3500 3400
200
300
BROOKFIELD VISCOSITY
40C C PS.
Figure 7. Adhesion increases with increase in polymer concentration with constant chain length Epon 1001 cured with equivalent amount of
4,4'-(diamino-)diphenylmethane VOL. 5 0 , NO. 6
0
JUNE 1958
925
---r
140
