Ind. EW. Chem. RM. Res. Dev. 1904, 23, 572-581
572
m, I 1. ,
?
.
.: :.... .
,,.,$;I ....
. . ' . .,I ,, , ,
I
p$F-y$?;,. ! * :.3!.. ,
Figure 4. Scanning electron micrographs of the surfaces of paints 4 (left) and 7 (right), dried at rmm temperature.
is generally sasumed to be gmater than the observed trend, SEM studies given in Figure 4 showed a slight reduction in surface sealing with increasing phtizicer concentration. This reduction may be attributed to either reductions in the tensile strength of the polymer binder due to increased plasticization or the premature coalescence of the small film forming particles in the channels needed for particles to reach the film surface. Since reductions in surface sealing were observed, premature coalescence of the particles is believed to play the major role in the decreased scrub resistance. Conclusions The present experimental study testing the exclusion mechanism investigated hy the authors in "model" latex films in formulated microvoid paints demonstrates that improved physical properties may be accomplished involing this mechanism. The efficiency of the exclusion of small particles, however, is limited by nonuniformly sized voids and channels reducing the packing efficiency of larger
particles as well as blocking passage of the smaller particles. Another factor limiting the exclusion mechanism is the premature coalescence of the small film-forming particles in the channels needed for particles to reach the film surface. The coalescence effect was further enhanced by the addition of external plasticizing agents that lower the temperature needed for film-forming particles (large and small) to coalesce in the channels at the beginning of the second falling rate drying period. This prohibits particles in the interior of the film from pushing to the air/paint film interface. Increasing the small particle/large particle number ratio provided increased scrub resistance at the expense of opacity by filling greater numbers of voids within the film. Optimization of the process would involve balancing the number of small particles in the system needed for adequate scrub resistance with opacity properties provided by the microvoids. Acknowledgment We gratefully acknowledge the contributions of Dr. Daniel F. Herman and the support of NL Industries and The Emulsion Polymer Institute, Lehigh University. Registry No. Ti02, 1346867-7;polystyrene (homopolymer), W3-53-6;(vinyl acehte).(ethylacrylate)(coplymer),25190-97-0. Literature Cited Bradfad, E. 8.; VandertoH. J. W.; Alhey. T. J. C&WScI. 1956. 11. 135. owbin. 0.P. m.0. o b w a ~Lehgh . university. bthbk" PA. 1980. El-Aasser, M. S.: Iqbai. S.; VacdehH. J. W. " M I O M acd Inlerfece S c ! e n ~ " .VCi. V Academic Press: New Ywk. 1976: p 381. Seiner. J. A. I&. Eng. me" W .Res. &v. 1978'. 17.302. VandwhoH, J. W.; Bradtmd. E. B. P a p k 1973. 27. 52.
Receiued for reuiew March 14, 1984 Accepted July 26, 1984
Improving Adhesion between a Segmented Poly(ether-urethane) and a Fluorocarbon Copolymer Coatingt D. Mark Hofhnan; Comb M. Walkup. and Ing L. Chlu hswrencs
L b "
NawOnal Laboratory. UvSnroe. CaMomia 84550
A moisture barrier coating of KeCF 800, developed at U N L to reduce uranium corrosion. had to be bonded to a porous ceramic. The adhesive could not bond too slrongiy or react with the coating and jeopardize its barrier properties. We studied methods of improving adhesion to the KeCF coating. Silane and titanate coupling agents
and a fluwocarbon surfactant were somewhat effective at increasing adhesion dapending on the application procedure. X-ray photoelectron spectroscopy (XPS) was used to demonstrate the presence of fluorosurfactant at tha fracture intMace. Postcuring at elevated temperatures (85 OC) also significantly improved adhesive strength to the fluorocarbon coating. This was anributed to thermal acceleration of interfacial diffusion of the urethane adhesive into the fluoropolymer surface.
Introduction Recently the polymer group at Lawrence Livermore National Laboratory (LLNL) developed a fluoropolymer barrier coating besed on the copolymer Kel-F 800 Walkup, 1983) produced by 3M Corporation, to reduce uranium 'Work performed under the auapicea of the US.Department of Energy by Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
corrosion due to moistwe. Epoxy (Hammon and Althouse, 1976) and poly(urethane) (Hammon et al., 1980. Hoffman, 1981,1983) adhesives, developed at LLNL. were used to bond Kel-F coated uranium parts. Two engineering requirements were placed on the adhesive/coating interface: (1) the fluoropolymer's moisture barrier properties must be retained to protect the uranium and (2) failure must not m r at the Kel-F/uranium interface since this defeats the purpose of the barrier coating. Further requirements of the adhesive were that the ultimate bond strength be
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 573
Table I. ComDositions of Urethane and Epoxy Adhesives % curing agent % type resin 74 XU-205 100 urethane Polvmee 2000 Hyiene W 26 (Desmodur Ma) 12 88 R/CA ratio 6.7 Versamide 140 94 epoxy Epon 871 5.5 93.3 DETA ERE 1359 0.5 FC-430 (Heloxy WC-69") 41.5 R/CA ratio 58.5 a Indicates that the original material used in this formulation is no longer available and has been replaced in current formulations by the material in parentheses.
near 10.3 MPa (1500 psi); thermal stability was required over the military temperatures range (-54 to 74 "C); and adhesive failure must occur without damage to the barrier coating. The epoxy adhesive, Explostik 473, contains diethylene triamine which attacks the Kel-F surface under ambient conditions. In butt tensile joints with Explostik bonded to Kel-F coated uranium and aluminum, failure occurs at the Kel-F/metal interface. Since the epoxy system failed both engineering requirements, it could not be used for this application. However, because the epoxy system failed at the Kel-F/metal interface, it proved useful for examining the lot-to-lot variation in the coating to metal strength and for coating formulation studies. On the other hand, the poly(ether-urethane) failed adhesively at the Kel-F interface with very low tensile strength. This paper describes our attempts to understand and improve adhesion between the fluorocarbon copolymer coating and the poly(ether-urethane) adhesive. One method which suggested itself almost immediately from numerous trade journal advertisements was the use of modifying additives. Although a wide variety of additives are known, no clear methodology was available for selection of either the most promising additive or the optimal application procedure. We examined six additives: a surfactant, three coupling agents, and two surface treatments. Improved adhesion to the coating was obtained with each additive but depended markedly on the application procedure. Coupling agents tend to be most effective when applied directly to the coating surface. This would require several extra steps in production, a definite disadvantage. Treating the surface with aliphatic amines increased the bond strength between the adhesive and coating above that between the coating and the metal, and this was unacceptable. Because adding the fluorocarbon surfactant to the prepolymer or curing agent improved adhesion by a factor of 2 and did not require extra steps in the application procedure, it was proposed for use in production. Major improvements in adhesion of the surfactant modified polyurethane were observed with changes in coating procedures and by postcuring the adhesive. Drying time and coating solvent composition had significant effects on the adhesive strength. This was due to removal of residual solvent from the coating. Postcuring at elevated temperature generally improved bonding. This appears to be associated with improved compatibility and increased diffusion at the adhesivelcoating interface. The final recommended procedure for improved bond strength was addition of the fluorocarbon surfactant FC-430 to the urethane prepolymer, curing at ambient temperature for 24 h, then postcuring at 85 "C for another 24 h. Experimental Section 1. Materials Formulations. Several papers have been published on the preparation and characterization of
Table 11. Composition of Kel-F-800 Coating Solutions i.d. no. % solids % ethyl acetate % xylene EA/Xy 1 28.6 67.8 3.6 95:5 2 9.1 86.4 4.5 955 3 41.1 55.9 3.0 95:5 4 28.6 70.0 1.4 98:2 5 28.6 65.0 6.4 91:9 60.5 11.5 84:16 6 28.0 7 38.0 56.4 5.6 91:9 72.0 0.0 100 8 28.0 60.0 6.7 89: 11 9 33.3 Table 111. Surfactants (S), Coupling Agents (CA), and Surface Treatments (T) Used to Improve Adhesion to Kel-F-800 trade name type structure mfgr FC-430 S fluorosurfactant 3M A-1100 CA aminosilane ucc A-187 CA epoxysilane ucc Kenrich KR-55 CA titanate DETA T triamine GE TETA T tetramine Y"
Halthane 88-2 polyurethane adhesives (Hammon et al., 1980; Hoffman, 1981, 1983; Larsen, 1978; Althouse and Hetherington, 1979). Table I lists the chemical composition and prepolper/curing agent ratio for this system. The Explostik 473 epoxy adhesive composition and ratio of resin to curing agent are also given in the table. This adhesive is less well characterized but discussions of its formulation and application procedures are available (Hammon and Althouse, 1976). The coating formulations were developed by C. Walkup. Those used in this work are given in Table 11. The use of ethyl acetate rather than methyl ethyl ketone eliminates the need for an epoxy primer (3M, 1977) and results in excellent film adhesion to uranium and aluminum. Xylene was used as a cosolvent to prevent skinning and reduce sagging of the coating. Good coatings were dried at 85 "C for 1to 4 h in forced-air ovens after a short ambient air dry of 10-40 min. Bubbles in coatings with higher xylene content were reduced by stepping the temperature to 85 "C in 5" increments every 10 to 20 min. When the coated aluminum or uranium test specimens were covered with a small plastic beaker to reduce volatilization, bubbles in 25 to 50 pm (1 to 2 mil) films were almost eliminated. Increased coating thickness can be obtained by increasing the polymer concentration or by multiple coats. We found that drying each coat before applying another worked best. Six additives, a surfactant (S), three coupling agents (CA), and two surface treatments (T) listed in Table 111, were used to improve urethane adhesion to the fluoropolymer. Four methods of application were studied: (1) direct application to the test fixture by wiping with a soaked cloth, waiting 1min, and wiping dry; (2) immersion of the test fixture in a primer of 5% additive in alcohol for 5 min and wiping dry. After either of the above methods the Kel-F coated fixtures were dried at 60 "C for 1h in a forced air oven or at 100 "C for 1h under vacuum. In methods (3) and (4) additives were directly incorporated into the prepolymer or curing agent, respectively, at 0.2 % by weight. Premixed prepolymer, curing agent and appropriate additive were mixed in 50-g batches by hand for at least 3 min, and then degassed by centrifuge for 1 min and applied immediately to both sides of the fixtures or puddled on one side using an acid brush. Fixtures were closed immediately after application of the adhesive (except as noted) to a 0.18-mm (7 mil) bond line as determined by shims. In most cases the adhesive was allowed to cure at
574
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I
1.5" AI right cylinder
-0.001-2'' Kel-F-800 -0.007" Adhesive
1
I
1!5" AI rightcylinder 1W
1 Figure 1. Schematic of butt tensile test specimen configuration.
ambient temperature for 16 h or more and subsequently postcured at 60,74, or 85 "C for 24 h in the fixtures. The specimens were cooled to ambient prior to testing. 2. Mechanical Testing. Anelastic behavior as a function of temperature for these adhesives was measured on a Rheometrics Mechanical Spectrometer (RMS) Model 7200 using forced torsion f&ures for redangular specimens approximately 6.5 X 1.2 X 0.32 cm (2.5 X 0.5 X 0.125 in.) (Rheometrics, 1974). An LSI-11/23 microprocessor system from Digital Equipment Corporation was used to control the experiment and record the data. Tensile breaking stress was measured on an Instron Model 1350 hydraulic test machine using butt tensile specimens. A second LSI-11/23 microprocessor was used for data acquisition in this test. Butt tensile specimens consisted of aluminum right cylinders with contact areas of 6.45 cm2(1inq2).The Kel-F 800 coating was applied to one surface. Single coat thicknesses, as measured by light section microscopy, averaged about 38 pm. Mechanical mounting fixtures holding up to 10 specimens maintained a 0.18-mm (7 mil) bond line between the Kel-F coated surface and the second aluminum right cylinder (see Figure 1). This configuration is similar to that recommended in ASTM D-897 (ASTM, 1982). Results and Discussion 1. Influence of Coating Formulation and Application Procedures on Adhesion. Two criteria for the Kel-F coating/adhesive combination were used as dependent variables to be optimized by adjustment of formulation and application parameters. First, the Kel-F coating must adhere well to the metal. Since bonding with Explostik 473 epoxy caused failure between the Kel-F and the metal, the epoxy was used to test the dependence of metal bond strength on coating formulation. Second, the strength of the bond between polyurethane and Kel-F should not be reduced. Three problems affecting the coating adhesion to uranium and bondability to the polyurethane were encountered during the development of the coating formulation and the application procedures. (1) The xylene added to ethyl acetate as a cosolvent to prevent sagging and skinning tended to remain in the coating after drying. (2) Because of the inverse relationship between moisture permeability and barrier coating thickness, formulations with higher solids content were attractive. However, the solvent also had to diffuse out of the coating without forming bubbles. Coating concentrations between 33 and 28% by weight produced 25 to 50 Mm (1 to 2 mil) thick Kel-F films of good uniformity with few or no bubbles. (3) Multiple coats for increased thickness of Kel-F worked best if the sample was dried for at least one hour at an elevated temperature between coats.
0 95
0 90
0 84
ow [ethyl acetale)
Figure 2. Bond strength of KeLF 800 to aluminum as a function of xylene concentration in the cosolvent. Circles and crosses indicate that the coating was dried 1 h or 4 h, respectively, at 85 OC. The Explostik epoxy resin formulation was cured at least 24 h at ambient then postcured 24 h at 85 "C.
The effect of cosolvent composition on the Kel-F coating to aluminum bond, tested using the epoxy adhesive Explostik 473, for the formulations listed in Table 111is shown in Figure 2. Coatings dried for 1 h at 85 "C (indicated by circles in the figure) tend to have lower strengths than those dried 4 h (indicated by crosses). The adhesive was cured 24 h at ambient and postcured 24 h at 85 "C in all cases. The simplest explanation of these results is that residual xylene in the coating acts as a plasticizer to decrease the modulus and adhesive strength of the Kel-F coating. We originally thought that adhesion of the Kel-F to aluminum might be influenced by the solvent composition since it is known (Folks and Mostafa, 1978) that different solvents affect the amount of polymer deposited on specific substrates. However, once the solvent has been removed at temperatures well above the polymer's glass transition, equilibrium chain statistics (Flory, 1953) suggest that the mean-square end-to-end distance of the polymer chain in pure polymer will be independent of the solvent from which it was cast. The data in Figure 2 seem to indicate that plasticization caused by the cosolvent xylene was responsible for a slight decrease in bond strength with increasing xylene concentration and the above-mentioned improvement of adhesion with drying time. This conclusion is supported by literature evidence that xylene tends to be retained by polymer coatings to a greater degree than ethyl acetate (Newman and Munn, 1975). Incomplete removal of residual xylene resulted in bond strengths of 0.6 to 1.0 MPa (100 to 150 psi) for FC-43O/urethane adhesive when the coating was dried at ambient for 24 h. Drying the Kel-F for 1 h at 85 "C and postcuring the adhesive at 85 "C for 17 to 24 h increased the bond strength to approximately 9 MPa (1300 psi). All failures of the bond between the polyurethane and Kel-F were in the adhesive. We recommend, if possible, that no xylene be added to the coating formulation. This necessitates the use of a partial enclosure around the coated part to reduce the rate of ethyl acetate evaporation and to eliminate bubbles. If xylene cosolvent must be used, we strongly suggest that a minimum drying time for each coat of 1h at 85 "C be allowed prior to bonding. The effect of solids concentration was studied briefly. We prepared solutions containing 10, 28, and 40% Kel-F in a solvent mixture consisting of 95% ethyl acetate and 5% xylene. These were applied to aluminum butt tensile cylinders, dried at 85 "C for 1h, and allowed to return to ambient before being bonded with the epoxy adhesive. Coatings were characterized by thickness, number of bubbles per square inch, and bond strength. Bond strength and bubble concentration depended inversely on
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4. 1984 575
Bond strength (piX 10-31 0 Thickness (mils)
2.0-
Control
-
FC-430 A-187 A-1100 KR-55
-
TETA
1 2 3 4 5
15
6
5 0.5 3
0
X solids by wi.
Figure 3. Kel-F 800 coating thickness, average number of bubbles per square inch of coating, and bond strength depended on solids content. Bond strength was reduced as the number of bubbles (defects in the coating) increased at higher solids content.
solids content while coating thickness was directly proportional to the Kel-F 800 concentration in the solution as shown in Figure 3. Clearly, the stress concentrating effect of bubbles in the coating reduced the adhesive strength. Higher xylene content also tended to increase coating thickness and bubbles. Lower solids content yielded higher strengths but unacceptable coating thickness. Coating thickness and bubbles appeared to be more strongly dependent on solution viscosity and volatilization than on solids content at concentrations below 33% solids. Multiple coats resulted in thicker polymer deposits, but bond strength of Kel-F to aluminum tended to drop, probably because of residual xylene. Double coats dried 1h at 85 "C between coats gave higher bond strengths than those with only one dfying after the final coat even though final thicknesses were comparable. Optimal solids concentration was 28-33% by weight. 2. Effect of Additives on the Adhesion of Halthane 88-2 Segmented Polyurethane. A fluorocarbon surfactant, two silane coupling agents, a titanate coupling agent, and two amine surface treatments were used to modifyimprove the adhesion of the urethane to Kel-F 800 coated aluminum. Figure 4 is a bar graph of the average butt tensile strength of the polyurethane adhesive with various additives. These adhesives were all cured at ambient for 24 h then postcured at 85 "C for 24 h. The surfactant, FC-430, was most effective when added to the prepolymer or curing agent. The coupling agents were most effective when applied directly to the surface or as primers from 5% alcohol solutions. Primary amine surface treatments to the Kel-F coating increased the bond strength above that of the coating itself and caused failure to occur between the Kel-F and aluminum fixture. When added to either the prepolymer or curing agent at concentrations of 0.25% by weight, the surfactant FC-430 produced at least a twofold increase in bond strength to Kel-F 800. Average bond strengths of 3.4 MF'a (500 psi) increased to 6.9 to 9 MPa (1000 to 1300 psi). Above 0.25% the additive phase separates in the cured polymer and is believed to be less effective. Little is known about the mechanism of this additive for improving adhesion. Because of its effect on urethane adhesion when incorporated directly into the prepolymer or curing agent, this additive was ultimately selected for further evaluation and study. The results of X-ray photoelectron spectroscopy (XPS) studies on the urethane/FC-430 system will be discussed in section 3. Basically, the FC-430 additive
prepolymer
In XU-205
Direct wipe
As primer
Figure 4. This bar graph shows the average butt tensile strength of polyurethane adhesive 88-2 bonded to Kel-F 800 coated aluminum modified with coupling agents, surfactants, or surface treatments. Each additive wa8 applied to the Kel-F coating directly or from an alcohol primer. Treated Kel-F coatings were dried at 60 O C in forced air ovens for 1h. Additives were also incorporated into the adhesive prepolymer or curing agent. Discussions of the treatment procedures are detailed in the text.
provided compatibility in the interface between the polyurethane adhesive and the chloro-fluoro copolymer. Interfacial diffusion is the simplest interpretation for the enhanced adhesion consistent with XPS results. Three coupling agents were tested using four application procedures. Union Carbide siloxane coupling agents A-187 and A-1100 and Kenrich Petrochemicals titanate coupling agent KR-55 were dissolved in the prepolymer or curing agent, applied directly or as 5% alcohol primer to the Kel-F 800 surface. Figure 4 also shows the average tensile strength for the 88-2 adhesive with these additives, cured at ambient for 24 h then postcured 24 h at 85 "C. Only direct application of the epoxysilane (A-187) and the alcohol primer for the aminosilane (A-1100) produced average bond strengths to Kel-F 800 above 10.3 MPa (1500 psi). KR-55 was also quite effective both as a primer and directly applied to the barrier coating. None of these coupling agents were as effective as the surfactant FC-430 when incorporated into the components of the adhesive. Clearly, the method of application played a significant role in the effectiveness of the silane coupling agents. The bond strength changed by almost a factor of 2 depending on whether the silane was applied directly or as a primer and the epoxysilane behaved exactly opposite of the aminosilane. In a second set of experiments coupling agents were added to the adhesive system already modified with FC-430 in the hope that improved bond strength would result. These specimens were cured at ambient 24 h, but postcured only 17 h at 75 "C. Further, the direct wipe and primer applications were dried under vacuum at about 100 OC rather than in forced air at 60 OC. Average bond strengths changed with the various additivies and FC430/urethane as shown in Figure 5. The major difference in these results was that the bond strength of the urethane modified only with aminosilane was highest when the coupling agent was applied to the Kel-F coating as a primer, but when the aminosilane and FC-430 were combined, direct application of the coupling agent gave the highest bond strength. In other words, the bond strength improvement was reversed for A-1100 applied to Kel-F directly or from methanol solution depending on the method of drying the coupling agent (compare Figures 4 and
576
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 2.0
-*
1 5
r a
1.0
d
"
E
G,
0.5
0 I"
In
arepolymer
XU-205
Direct wipe
AS primer
Figure 5. This bar graph shows the average butt tensile strength of polyurethane adhesive 88-2 with 0.25% FC-430 surfactant bonded to Kel-F 800 coated aluminum modified with the identical additives as in Figure 4. Drying procedure for application of additives to the coating surface in this experiment was 1 h under vacuum at 100 "C compared to 60 OC in forced air for Figure 4.
5). A recent review (Plueddemann, 1982) has suggested that the reaction products of a silane coupling agent can vary depending on the presence of moisture and the temperature. The orientation of the reactive functional group and changes in modulus of the interphase layer due to the degree of cross-linking can be used to rationalize our results. A-1100 is y-aminopropyltriethoxysilane(Union Carbide, 1976, 1979, 1983). Since the amine-isocyanate reaction would reduce its effectiveness (Swanson and Price, 1973), this coupling agent was not added to the prepolymer. When A-1100 was added to the XU-205 aromatic diamine curing agent, no indication of adverse reaction was observed. Although several claims have been made regarding improved adhesion on addition of aminosilanes to the curing agent of a urethane (Union Carbide, 1976, 1979, 1983; Swanson and Price, 1973; DeLollis, 1973, 1977), we found this procedure ineffective for the 88-2 adhesive. Direct application of A-1100 to the Kel-F surface produced a poor bond, no better than the control (88-2 with no additives). When the aminosilane was applied as a 5% primer from methanol for 20 min and dried at 60 "C in air, a remarkable increase in bond strength was observed. One plausible explanation of this might be that the methanol removed low molecular weight oligomer and contaminants which enabled the aminosilane to react with the coating surface. However, this is inconsistent with the results observed when FC-430 and A-1100 are used together. In this case the direct application of A-1100 yields higher bond strength to Kel-F than application from a primer. A better explanation, consistent with literature evidence (Plueddemann, 1982),is that the effectiveness of a coupling agent depends on both the chemical reaction of the functional group with the substrate and the mechanical interaction or modulus of the interphase formed by the coupling agent, adhesive, and adherend surfaces. The chemical coupling consists of physi- and chemisorption of the coupling agent onto the substrate and chemical bond formation with the adhesive or adherend. The mechanical properties of the interphase are related to the degree of cross-linking occurring due to silane hydrolysis. Silane hydrolysis is catalyzed by moisture and heat. If a moderate amount of water is present in the MeOH or if MeOH can also catalyze hydrolysis, a moderately cross-linked "tarry" silane interphase would be viscous and self-healing and good bond strength should result. Also, more amine groups would be available to react with the adhesive in the primer
process. Direct application of the aminosilane would lead to unhydrolyzed monomer or low molecular weight oligomer at the interphase since only limited amounts of catalytic moisture would be present. Furthermore, A-1100 forms strong hydrogen bonds between the amino group and the silanols (Schrader and Block, 1971; Boerio et al., 1980, 1983; Chang et al., 1980; Ishida et al., 1982) which hinder or prevent its reacting with the adhesive. Bonding through a monomer or oil interphase may be disrupted by surface hydrolysis of only a few bonds. This along with the unreactivity of the aminosilane monomer explains why it produced poorer bonding when forced-air dried at 60 "C than did the tarry, partially hydrolyzed silane interphase deposited from 5% primer. At 100 "C under vacuum, the primer-deposited aminosilane formed a visible brittle film which we attribute to a high degree of cross-linkingat the elevated temperature. The higher temperature and vacuum for removal of moisture generated in the silanol hydrolysis would tend to push this reaction to higher conversion. In the direct wipe, A-1100 samples dried under vacuum at 100 "C, the hydrolysis reaction would proceed, but not to sufficiently high conversion to become brittle, in which case the bonding interphase would be excellent. We believe that the extent of reaction of the aminosilane interphase layer determines the effectiveness of the bond between the polyurethane and Kel-F 800. The availability of the amino group to the adhesive and adherend as a reactive site and the mechanical properties of the interfacial layer depend on the extent of reaction of the silane coupling agent. The kinetics of this reaction are determined by concentration, catalyst, and temperature. This explains how, by appropriate control of temperature and catalyst, a good bond could be obtained from either direct application of primer-deposited aminosilane. A-187, y-glycidoxypropyltrimethoxysilane,was most effective when applied directly to the Kel-F coating. This coupling agent has been shown to improve adhesion of polyurethanes to some substrates (Swanson and Price, 1972; DeLollis, 1977; Walker, 1983; Dow-Corning, 1981). 3M recommends epoxy primers to improve Kel-F coating adhesion to metals (3M, 1977). Since epoxy/aromatic diamine reactions are well known, this coupling agent was not added to the curing agent. The bond strength of the urethane to Kel-F bond was not improved by either addition of A-187 coupling agent to the prepolymer or treatment of Kel-F with 5% coupling agent in ethanol. There is some viscosity evidence which suggests that epoxysilanes react slowly with polyurethane prepolymer (Swanson and Price, 1972, 1973). If this were the case in the 88 prepolymer, no interphase layer would be formed and bonding would not improve. In fact, our data show the worst bonding behavior for epoxysilane in the prepolymer of all the application techniques. Apparently there is a specific interaction between the epoxy group and the Kel-F 800 surface because direct application of A-187 to the surface results in the most improved bond strength and least scatter of data, with or without FC-430. Average bond strengths of 11.2 MPa (1620 psi) with FC-430 and 10.7 MPa (1550 psi) without FC-430 were obtained. The actual mechanism of this interaction is not known. However, it would seem to involve an epoxy functionality which is reasonably unhindered by polymerization of the silane since primer coatings using A-187 were ineffective as adhesion promoters. Kenrich Petrochemicals recommended (Kenrich, 1979) three of their titanate coupling agents, KR-41B, KR-55, and KR-44, as adhesion promoters for polyurethanes. KR-55, tetra-(2,2-diallyloxymethyl-l-butoxytitanium di-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
(ditridecyl) phosphite, was used at slightly less than 1% as recommended in the product literature. Kenrich recommended addition of the titanate to the poly01 component (in the 88 prepolymer). Since no reaction was expected from either the prepolymer or curing agent, KR-55 formulations were made with the coupling agent in either the prepolymer or curing agent. The titanate was most effective when applied as a 5% primer in methanol or directly to the Kel-F 800 surface and dried in a forced air oven for 1 h. Contrary to Kenrich literature, a slightly stronger bond was obtained when KR-55 was incorporated into the aromatic diamine curing agent rather than the prepolymer. Addition of the titanate to the adhesive formulation gave only slightly improved adhesion. When FC-430 was incorporated in the adhesive, KR-55 was less effective as a primer or direct wipe. Perhaps this was because of modification of the Kel-F surface by KR-55 to alter the ability of the surfactant to reduce surface tension. Within experimental error, there is no difference between the FC-430/adhesive system formulated with or without the coupling agent. Only when the KR-55 was applied to the KeEF and when the adhesive contained no s u r f a c h t was this coupling agent effective. Unfortunately, only limited information is available in the literature on titanate adhesion promoters (Monte and Sugarman, 1983) and further study will be necessary to elucidate the details of the adhesion improvement mechanism of KR-55. When the Kel-F 800 coating was etched with primary amines, exceptional bond strengths between the polyurethane and the coating were obtained. Failure generally occurred at the interface between Kel-F and aluminum. Bond strength increased linearly with the logarithm of the immersion time in triethylene tetramine from 15.3 MPa (2220 psi) for 1s immersion to 17.7 MPa (2570 psi) for 10 min immersion (Childress, 1983). This would seem to indicate that the amine reacted with the KeEF surface and probably altered its permeability. Note that the failure surface was no longer that between the adhesive and fluoropolymer coating, but that between the Kel-F and the aluminum or uranium. The cross-linking reactions of fluoroelastomers with amines are well-known (Paciorek et al., 1960; Paciorek, 1972; Smith, 1961). As shown in Figure 4, the strongest bond between Kel-F and the polyurethane was obtained with primary amine surface treatment. Unfortunately, because of the restriction that the coating to metal interface remain essentially undamaged, this surface treatment could not be used. In general, the coupling agents used in these tests were most effective as surface treatments to the Kel-F coating. The highest, must consistent bond strengths were obtained from direct application of the A-187 epoxysilane to the Kel-F surface. This coupling agent was effective whether or not the 114% FC-430 surface had been formulated into the poyurethane. The aminosilane was also effective as a primer when dried at 60 "C and when directly applied and dried at 100 "C under vacuum, but the scatter in the data was greater for this coupling agent. KR-55 also improved bond strength over the unmodified urethane when applied directly to Kel-F 800. The primary amine surface treatment was more effective at improving bond strength between Kel-F and the polyurethane than either the coupling agents or FC-430, but could not be used because the adhesive strength exceeded the bond strength between coating and metal. Because significant improvement in adhesion was obtained by direct addition of FC-430 to the prepolymer or curing agent, this system was selected for further study. Direct incorporation of the additive into the two-part adhesive was preferable because it eliminated
577
BE in eV
Figure 6. X-ray photoelectron spectroscopy C 1s data were taken from thin films of 88-2 polyurethane with 0.25% FC-430fluorosurfactant (dotted line) and without the additive (solid line). This data implies that the FC-430migrated to the adhesive surface. XPS C Is data were also taken from Kel-F 800 copolymer film.
the surface treatment steps in the bonding process. 3. Surface Studies. Fischer (1983) ran X-ray photoelectron spectroscopy (XPS) measurements on films of Kel-F 800, Halthane 88-2, and Halthane 88-2 with FC-430. Since there is a relationship between the charge state of the atom and the binding energy of its core electrons, we expected to be able to identify the chemical species on the surface from XPS data (Reilley et al. 1982). Figure 6 shows XPS results for polyurethane adhesive films with and without FC-430. The carbon 1s binding energies for the urethane adhesive with FC-430 surfactant contained at least two extra peaks corresponding to di- and trifluorosubstituted carbon from the surfactant. Because the electron escape depth is only about the first 5 nm of surface, the presence of large di- and trifluoro-substituted carbon peaks clearly indicates the surfactant FC-430 concentration on the adhesive surface is greater than would be expected for a uniform distribution of 0.25% additive. Obviously, the surfactant migrated to the adhesive surface. A small peak near 288 eV was also observed. Unfortunately, both vinyl fluoride and urea C 1s binding energies are in this range. Since the aromatic and aliphatic carbons show a slight increase at the expense of the ether soft segment carbons in the XPS data, it is possible that more hard segments are on the FC-43Oladhesive surface. The Kel-F 800 XPS results (also shown in Figure 6) showed a ratio of vinylidene fluoride to chlorotrifluoroethylene of approximately 1 to 3, in agreement with conventional chemical analysis (Cady and Caley, 1977). XPS measurements from the failure surfaces of the 88-2 polyurethane adhesive bonded to Kel-F 800 were made after the adhesive had been carefully removed from the aluminum test fixture. Figure 7 shows the results from: (1)Halthane 88-2 cured at ambient then postcured at 85 OC for 24 h (3.9 MPa or 560 psi bond strength), (2) 88-2 plus 0.25% FC-430 surfactant cured at ambient (5.0 MPa or 730 psi bond strength), and (3) 88-2 with FC-430 cured at ambient 24 h then postcured at 85 OC for another 24 h (bond strength of 12.3 MPa or 1780 psi). Examination of the failure surfaces of the polyurethanes from butt tensile specimens showed areas of different light reflecting properties, one shiny and the other dull. Distinctly different spectra were obtained from dull and shiny areas.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
578
2K
- =1
-.-.....
1
'
1
'
1
88-2 (shiny) s 2 a 88-2 + FC (shiny) * * rr2b 88-2 t FC (dull) =3a 88-2 t FC t 8 5 C PC (shiny,{
: :
::
i
+ + + =3b 88-2
+
FC + 85'C PC (dull1
: :
1
1
I
E1
280
300
,
I
4
p3-I 1
2i-
BE in eV
Figure 7. XPS C 1s data were taken from failure surfaces of 88-2 adhesive with and without FC-430 from dull and shiny regions on the surface. The difluoro-substituted C 1s peak intensity on the adhesive surface correlated directly with the bond strength (see text for further discussion).
The dull areas containined significantly higher concentrations of the carbon 1s peak from difluoro-substituted species implying that large amounts of vinylidene fluoride and chlorotrifluoroethylene from the Kel-F copolymer remained on the dull surface. We interpret this to mean that the FC-430 not only improved wetting characteristics between the Kel-F and polyurethane adhesive, but also made the two polymers more compatible in the neighborhood of the surface. This compatibility allowed them to interpenetrate, improving the bond strength. The XPS of Halthane 88-2 failure surfaces without FC-430 had essentially two carbon 1s peaks corresponding to ether linkages in the soft segments and mixed aliphatic and aromatic carbons. There was also a small 288 eV peak (see Figure 7). This spectrum is almost identical with that of the 88-2 film spectrum in Figure 6. This would imply that very little interaction occurred between the adhesive and adherend without the surfactant being present even with the standard postcure. The addition of FC-430 resulted in 3 new peaks, di- and trifluorocarbon peaks and a peak at 288.0 eV which could either be urea hard segment or -CHF- groups. The shiny regions of the failure surface gave XPS spectra#wth only slightly more difluorocarbon peak intensity than the film of unbonded Halthane with FC-430. The dull regions, on the other hand, showed difluoro peaks broad enough to conceal a -CFCl- peak and too large to result from surfactant alone. The postcured dull regions showed difluorocarbon intensities larger than any other peaks and the spectrum was very similar to that of Kel-F 800 with some hydrocarbon on the surface. This would imply that, especially in the dull regions, the adhesive has migrated into or reacted with the Kel-F coating sufficiently to pull off some of this fluoropolymer in the process of fracturing. If the urea hard segment migrated to the surface or if this type bond is formed between the Kel-F and aromatic diamine, the modulus of the interface would increase and bond strength should improve. If the FC-430 helped to improve interfacial compatibility between coating and adhesive, the bond strength should also improve. Clearly, an interaction between the adhesive and adherend occurred in the dull regions of the polyurethane failure surface. From the XPS measurements, we concluded that the mechanism of adhesion enhancement by the fluoro-
'
48 m
=
Halthane 88-2iFC-430
I
= Explostik
10
20
30
Time prior to application in minutes
Figure 8. Bond strength in 88-2/FC-430 polyurethane and 473 epoxy adhesive decreased with increasing time between mixing and application to the butt tensile fixtures. The 88-2/FC-430 passed through a maximum at about 10 min after mixing.
surfactant FC-430 was complex. It involved preferential segregation of FC-430 to the surface, improved surface wetting, and some form of chemical interaction between the adhesive and the fluoropolymer coating, especially in the dull regions of the failure surface. We proposed at least two mechanistic possibilities for the enhanced interaction between Kel-F 800 and Halthane 88-2 as a result of FC-430 addition. First, the surfactant must improve the Kel-F compatibility because significant amounts of difluorocarbon show up on the adhesive surface. Further, it is possible that chemical reactions between the aromatic diamine and Kel-F surface could form covalent bonds which would be stronger than either the fluoroelastomer or Halthane adhesive themselves. Since aromatic diamines are less reactive with fluoroelastomers at lower temperatures, this would help to explain the twofold increase in bond strength with postcure at elevated temperatures. Postcure at elevated temperatures would also improve interpenetration of the more compatible polymers since molecular motion in the Kel-F 800 copolymer would increase the higher the postcure temperature increased above the glass transition temperature (30 "C). We feel, therefore, that both improved compatibility at the interface and reaction with the curing agent are likely to be involved in bond strength improvement during postcure. 4. Influence of Application Procedure and Formulation of Polyurethane and Epoxy Adhesives on Adhesive Strength. Both the epoxy and polyurethane adhesives were tested to determine whether an optimum application procedure could be found. The mixing time, stoichiometry, postcure time, and temperature influenced adhesive strength in the polyurethane/FC-430 formulation. The optimum mixing time for the urethane/FC-430 adhesive was approximately 10 min. Application to either or both surfaces had no effect in either adhesive. The urethane/FC-430 formulation showed optimum strength when the amine/isocyanate equivalence ratio was 1.12, i.e., slightly excess amine. Postcuring time and temperature played an important role in both adhesives' bonding ability. Recommended preparation procedures for Halthane 88-2 (Hammon et al., 1980) were followed with FC-430 added to the prepolymer. Prepolymer and curing agent were mixed rapidly by hand in 88:12 weight ratio and centrifuged to remove entrapped air. This process took
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984 579 8,
!I
,
,
,
,
:
,
,
,
,
-c
+
f
E
8 1.0
1 t
t -
5
0
09
10
11
Haltham 88-ZFC-430 cured at ambient 24 hr
1.2
Equivalent ratio [NHI /[NCOI
Figure 9. Variation in 88-2/FC-430 adhesive strength with stoichiometry showed that slight improvement in strength occurred with slight excess of aromatic diamine curing agent. However, higher amine concentrations resulted in a dramatic reduction in strength.
about 6 min. The butt tensile strength as a function of time after mixing (see Figure 8) reached a maximum after 10 min. Application of 88-2 polyurethane adhesive to the test fixtures after 20 min or longer resulted in reduced bond strengths. Since the gel time for this adhesive (Hammon et al., 1980) was 45 min at room temperature, it was surprising that mixing time should effect the bond strength. Changes in the viscosity with time might reduce adhesive strength, but no minimum or maximum was detected in plots of viscosity vs. time curves during ambient cure (Hoffman, 1983). The effect of stoichiometry on bond strength is of interest, especially in the polyurethane, because production formulation errors or inadequate mixing can result in off stoichiometry adhesive. Also slightly excess isocyanate is known to improve adhesion between polyurethanes and some metal substrates (Reegan et al., 1962; Frisch et al., 1971; Liang and Dreyfuss, 1983). If interactions between the fluorocarbon coating and the aromatic diamine curing agent are an important mechanism for adhesion enhancement (Paciorek et al., 1960; Smith and Perkins, 1961), excess amine would be expected to improve the bond strength to Kel-F. The polyurethane adhesive as formulated contains a slight excess of isocyanate. As suggested above, increasing the amine/isocyanate mole ratio (see Figure 9 ) raised the bond strength between 88-2 and Kel-F slightly, from 8.5 to 9.2 MPa (1230 to 1330 psi). Bond strength reached a maximum at an NH/NCO mole ratio of 1.13 and then dropped off rapidly as more XU-205 was added. This implies that as much as 3.5 g excess amine in the 100 g adhesive formulation would slightly improve adhesion to Kel-F 800. This result supports our previous assertion that the aromatic diamine curing agent affected the bonding to the Kel-F 800 surface. When the isocyanate functionality was in excess, only a moderate reduction in bond strength occurred. This would seem to imply that the presence of excess amine is not especially critical. In fact, a large excess of amine in the curing agent did not improve adhesion. If some chemical cross-linking or reaction involving the XU-205 curing agent occurred at the urethane/fluoro-
0 75'C p0StC"re 0 60°C P O ~ ~ C Y R ) A Ambient cure
3
10
30
1w
3w
pastcure time in hours
Figure 11. 88-2/FC-430bond strength increased with time of postcure under isothermal conditions.
carbon polymer interface, the kinetics of that reaction would be expected to affect the adhesive strength. Our decision to cure at ambient at least 16 h prior to postcuring was due to recent evidence (Hoffman, 1983) from rheological measurements of undesirable side reactions when these polyurethanes were cured at elevated temperatures. The dependence of bond strength on postcure temperature for 88-2 polyurethane cured at ambient for 24 h then postcured at ambient, 60,74, and 85 "C for 24 h was Arrhenius-like (see Figure 10) with an activation energy from least-squares fit of about 22 kJ/mol. This low activation energy is indicative of a diffusion controlled reaction (Frank, 1950) and is too low to be considered evidence for reaction between XU-205 and Kel-F. We also examined the change in adhesive strength with time under isothermal postcuring temperatures of 60,75, and 85 "C. Figure 11 shows that there is an increase in bond strength with time which has a positive temperature coefficient typical of a chemical reaction. Unfortunately, as in most bond strength measurements, there is considerable scatter in the data. Although the trend toward improved bond strength with increasing postcure temperature is clear, the shape of the isotherm of bond strength vs. time is less clear. Poetcuring below 85 "C even for extended periods of time did not increase bond strengths over 8.3 MPa (1200 psi). The effect of postcuring temperature on the epoxy adhesive is shown in Figure 12. Postcuring at temperatures well below the glass transition temperature of the epoxy had no effect on adhesive strength. As the postcuring temperature approached the glass transition temperature, the bond strength increased and probably would level off
580
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 4, 1984
$1
-
20
lo
\ 20 40 60 80 100 Postcure temp j C i
Figure 12. Increasing postcure temperature of the 473 epoxy increased the adhesive strength only when the postcure temperature was above the glass transition temperature.
at postcure temperatures above the glass transition. The improvement in adhesion was associated with incomplete polymerization of the epoxy adhesive cured below its glass temperature. At lower cure temperatures, vitrification occurred before network formation was complete. This resulted in a more brittle epoxy network weakened by unreacted portions of the resin. Postcuring near the glass transition allowed these reactions to be completed and strengthened the network (Gillham, 1979; Enns and Gillham, 1983). Conclusions The epoxy adhesive generally produced failure at the coating to metal interface and at considerably higher strength than the polyurethane adhesive. This result was used to examine the effectivenesa of different Kel-F coating formulations on coating to metal adhesion. Increasing xylene concentration in the Kel-F cosolvent tended to reduce the bond strength of the coating to metal. We believe this was due to residual solvent remaining in the coating even after 4 h drying at 85 "C. Studies of formulation and application procedure for the epoxy showed reduced bond strength with increased time between mixing and application. Postcuring the epoxy bond was only effective when the temperature exceeded 75 "C, the glass transition of the epoxy adhesive cured at ambient. Because the strength of the epoxy adhesive exceeded that of the Kel-F coating to metal, this adhesive was unsuitable for our application. Our results indicate that adhesion of the HMDI, polyol, substituted aromatic diamine polyurethane to Kel-F 800 could be increased substantially by the use of small amounts of additives. The specific method of incorporation or application and subsequent drying procedures for the additives determined whether or not enhanced adhesion was obtained. The aminosilane coupling agent A-1100 was effective when applied as a 5% primer from alcohol and dried at 60 "C. When applied directly to the coating surface, A-1100 improved adhesion between the polyurethane and coating only when dried under vacuum at 100 "C. This difference is attributed to the extent of silanol hydrolysis caused by catalytic effects of moisture and temperature. The epoxysilane A-187 was an effective adhesion promoter only when applied directly to the Kel-F surface. The titanate coupling agent KR-55 was also effective at increasing the bond strength of the polyurethane when used as a primer or by direct application to the adherend. Direct surface etching with multifunctional primary amines produced the strongest bonds between the polyurethane and Kel-F and resulted in a failure at the interface between the Kel-F coating and the aluminum fixture, exclusively. Addition of 0.25% of the fluorocarbon surfactant FC-430 to the urethane prepolymer or curing
agent and an elevated postcure temperature of 85 "C for 24 h was ultimately selected as the best method for our purposes. We developed the 882/FC-430 adhesive system to meet design requirements for bonding to Kel-F 800. After examining several other approaches, this system best met the engineering adhesive requirements. X-ray photoelectron spectroscopy and formulation studies were used to try to understand the mechanism of adhesion improvement of the FC-430 fluorosurfactant. XPS indicated that the mechanism of adhesion enhancement of the FC-430 surfactant was complex, involving reduction of the work of adhesion, improved compatibility of the fluorocarbon copolymer surface with the adhesive, and perhaps some chemical interaction between the aromatic diamine curing agent and the Kel-F at elevated temperatures. Studies on formulation and application procedures for the FC-430/urethane and the epoxy adhesives indicated that mixing time postcure time and temperature, and stoichiometry were important factors in maximizing bond strength between the polyurethane and the barrier coating. Neither adhesive was affected by various application techniques to the coating. The improvement in bond strength with slight excess of aromatic diamine curing agent is believed to result from a reaction between the aromatic diamine and the coating surface at elevated temperatures. Because of the unreactive nature of the aromatic diamine, bonding through this interaction is less efficient than surface etching with TETA or DETA. Consequently, failure still occurred adhesively at the coating to adhesive interface. XPS results and dependence of bond strength on postcure time indicated that postcuring at elevated temperature compatibilized the KelF/FC-430/urethane interphase. This finding was also supported by the presence of Kel-F on the urethane failure surface. This implied that interfacial diffusion between the coating and adhesive had occurred to a depth of at least 5-10 nm. Improvement in bond strength with postcure temperature was weak, indicating that diffusion may be controlling this process. It may be that the main contribution of the FC-430 to bond improvement was to compatibilize the adhesive/coating interphase and promote interfacial diffusion. The surfactant additive FC-430 was preferred in our application because it raised breaking stress, and eliminated several steps involved in surface treatment procedures, but did not defeat the primary purpose of the Kel-F coating (to protect uranium metal from spallation due to moisture corrosion) by exceeding the coating to metal bond strength. Acknowledgment We wish to thank J. R. Humphrey and J. E. Hanafee for their support of this work; B. M. McKinley and A. T. Buckner for helping to assemble, coat, and bond many of the tensile specimens; S. B. Monaco and D. C. Johnson for the light section microscope thickness measurements; H. H. Whiting and his electronics group, especially F. J. Bancalari, for their assistance with the LSI-11/23 hardware; S.D. Koopman, D. W. Freeman, and S. M. Lanning for writing the data acquisition and plotting software; J. W. Fischer for running the XPS analysis; J. D. Bradley of the art department for translating our data into art; J. F. Carley for his fine editorial assistance, and last but not least J. E. Kervin and other members of LLNL's engineering group for their patience and encouragement. Registry No. (Polymeg 2000).(hylene W) (copolymer), 61641-57-4; (epon 871).(ERE 1359) (copolymer), 30939-39-0; kel-F-800,9010-75-7;FC-430,11114-17-3;A-1100,919-30-2;A-187, 2530-83-8; KR-55,64157-14-8;DETA, 111-40-0; TETA, 112-24-3; halthane, 88-2.
581
Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 581-586
Literature Cited Althouse, L. P.; Hetherlngton. N. W. “Study of the Aging Processes In Polyurethane Adheslves uslng Thermal Treatnmnt and Dlfferentlal Calorimetric, Dlelectrlc and Mechanical Techniques”; UCRL 52849, Lawrence Livermore National Laboratory: Livermore. CA, 1979. American Society for Testing and Materials “Annual Book of ASTM Standards”; Philadelphia, 1982; Vol. 22. p 220. Boerlo, F. J.; Armogan, L.; Cheng. S. Y. J. ColbU Interface Scl. 1980, 73, 416. Boerlo, F. J.; Wllliams, J. W.; Burkstrand, J. M. J . ColbU Interface Scl. 1983, 97, 485. Cady, W. E.; Caley, L. E. “Properties of KeK-800 Polymer”; UCRL 77897. Lawrence Livermore National Laboratory: Uvermore, CA, 1977. Chang, CH.; Ishida, H.; Kaenig, J. L. J. ColbU Interface Scl. 1980, 74, 396. Chlldress, F. Union Carbide Corp., Y-12 plant, Oak Ridge, TN, unpublished results, 1983. DeLollls, N. J. “Properties of Slllcones, Polyurethanes and Structural Epoxy Adhesives”; SLA-73-0385, Sandla National Laboratory: Albuquerque, NM, Oct 1973. DeLollls, N. J. “Adhesive Properties of Urethane Resin”; SLA-76-5225, Sandia Natlonal Laboratory: Albuquerque, NM, 1977. DOW Conring Gorp. “ S k W Coupww Agents”; Buktkr NO. 25012A-81, 1961. Enns. J. 6.; Qllham, J. K. J. Appl. P w m . Scl. 1983, 2 8 , 2831. Flscher, J. “ESCA Examination of the Halthane 88-2 Bonding Surfaces of KeK-800/Halttpne 88-2 bonds from Tenslle Test Specimens”; presented at the Annual DOE Adhesives Symposium, Livermore, CA, June 1983. Flory, P. J. “Principles of Polymer Chemktry”; Cornell University Press: Ithaca,NY, 1953; Chapter I O , 11, 12, and 14. Folks, F. M.; Mostafa, M. A. Ind. Eng. (2”.prod. Res. Dev. 1978, 77, 3. Frank, F. C. Roc. R. Soc.London, Ser. A 1950, 207, 586. Frlsch, K. C.; Reegen, S. L.; Rumao, L. P. A&. Urethane Scl. Techno/. 1971, 7 . 49. Glllham, J. K. P m m . Eng. Scl. 1979, 79. 676. Hammon, H. G.; Aithouse, L. P. “Adhesive Bondlng of TATB/KeCF-800”; UCID 17319, Lawrence Livermore Natlonal Laboratory: Livermore, CA, 1976. Hammon, H. 0.; Althouse, L. P.; Hoffman, D. M. “Development of Halthane Adhesives for Phase 3 Weapons: Summary Report”; UCRL 52943, Lawrence Livermore National Laboratory: Livermore. CA, 1980. Hoffman, D. M. ACS Symp. Ser. 1981, 772. 343. Hoffman, D. M. ACSSymp. Ser. 1983, 227, 169. Ishda, H.; Navlroj, S.; Trlpathy, S. K.; Fkgeraid, J. J.; Koenig, J. L. J. Polym. Scl.: Polym. phvs. Ed. 1982, 2 0 , 701. Kenrlch Petrochemicals, Inc. “Titanate Coupling Agents for Filled Polymers”; Bulletin No. KR-0278-7, 1979. Larsen, F. N. “Synthesis of Halthane 88 Prepolymer and Evaluation of X U 205 Diamine Cured 88 Repolymer Adhesive”; BDX 613-1035, The Bendlx Corpotation: Kansas City, MO, Sept 1978. Liang, F.; Dreyfuss, P. Coat. Appl. Polym. Scl. Roc. 1985, 48 19. 3M Company “KeCF Brand 800 Resin”; Technlcal Data Sheet Y-IKFBOO. 1977.
&b.
Monte, S. J.; Sugarman, 0. J. Cell. Pkrst. 1983, 19, 109. Newman, D. J.; Munn. C. W. Pmg. Org. Coatlngs 1975, 3 , 221. Paclorek, K. L.; Mitchell, L. C.; Lenk, C. J. J. Polym. Scl. 1960, 4 5 , 405. Paclorek, K. L. I n “Fluoropolymers”; Wall, L. A., Ed.; Wlley: New York, 1972; pp 291-315. Plueddmann, E. P. “Silane Coupling Agents”; Plenum Press: New York, 1982. Reegen, S. L.; Ilkka, G. A. I n “Adhesion and Cohesion”, P. Weiss, Ed.; Elsevler: New York, 1962. Rellley, C. M.;Evehart, D. S.; Ho, F. F A . In “Applied Electron Spectroscopy for Chemical Analysis”; Wlndawl, H.; Ho, F. F.-L., Ed.; Wiley-Intersclence: New York, 1982; pp 109-131. Rheometrlcs, Inc. Rheometrics Mechanical Spectrometer Operating Manual”; 1974. Schrader; M. E.; Block, A. J. Polym. Scl. Symp. 1971, 3 4 , 283. Smith, J. F.; Perklns, G. T. J. Appl. Polym. Scl. 1961, 76, 460. Swanson, F. D.; Price, S. J. Adhes. Age March 1972, 75, 26. Swanson, F. D.; Price, S; J. Adhes. Age June 1973, 76, 23. Union Carbide Corp. Silane Coupling Agents In Mineral Reinforced Elastomers”; Bulletln F-44715C, April 1979. Unlon Carbide Corp. “Organofunctlonal Silanes-A Profile”; Bulletin SIU-6A, May 1983. Unkm Carbkle Corp. “Union Carbide A-I10 Silane Adhesion Promoter”; Bulls tln F-41926, 1976. Walker, P. “An Update on Silane Coupling Agents”; presented at the Annual DOE Adhesives Symposium, Livermore, CA. June 1983. Walkup, C. Lawrence Llvermore National Laboratory, Livermore, CA, unpublished results, 1983.
Received for review May 21, 1984 Accepted June 25, 1984 This document was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial products, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government thereof, and shall not be used for advertising or product endorsement purposes.
Dispersion Coatings as Wear Protection of Components Subjected to Stress at Elevated Temperatures Martin Thoma mtu Motoren- und Turblnen-Union, Mijnchen GmbH, Dachauer Strasse 665, 08000 Mijnchen 50, West Germany
Eiectro- or eiectroiessdepositeddispersion coatings-a composite of coating matrix and embedded particiesprovide outstanding protection against wear. The knowledge of the coating properties, such as hardness, of changes in characteristics and decomposition, necessary for ensuring optimal applications, is presented together with the tribological characteristicsof the coatings at elevated temperatures. The degree of protection against wear, as well as the other properties of the coatings, is affected by temperature. Depending on the temperature to which the component is subjected, the optimal dispersion coating as protection against wear associated with reciprocating stress, such as NVP diamond for 20-400 O C or Co Cr203for 300-700 OC, can be recommended.
-+
Introduction Components which are subjected to wear accompanied by temperature stress need to be provided with enhanced antiwear characteristics by a surface coating, which must have good resistance to both wear and elevated tempera0196-4321/84/1223-0581$01.50/0
+
ture. In addition to high adhesive strength of the coatings, the essential technical and economic requirements made of the coatings and coating processes are low thermal and mechanical stressing of the component during coating, the possibility of local coating and lining, minimal rework, high 0 1984 American Chemical Society
