Contact Angle, Wettability, and Adhesion

11 The Relationship between Wetting and Adhesion

Downloaded by MIT on May 11, 2013 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch011

J. R. HUNTSBERGER Fabrics and Finishes Department Ε. I. du Pont de Nemours & Co., Inc. Wilmington 98, Del. Adhesion of polymers was determined as a function of temperature. The influence of the bonding times and temperatures indicates that the performance is established largely by the extent of wetting at the polymer -substrate interface. Considerations based on surface free energies show that most practical systems should exhibit complete wetting at equilibrium. The problem appears to involve establishing factors which retard or preclude wetting. Low substrate surface energy, high polymer viscosity, substrate topography, selective adsorption, and coacervation may be involved.

When two solid materials are brought together in such a way that intimate molecular contact between the phases is achieved, they should adhere strongly to each other. Many authors have shown that disper sion forces alone are sufficient to lead to high values for the work of adhesion; consequently, relatively good adhesion should be exhibited despite the chemical nature of the materials. In practice examples of very poor adhesion are frequently ob served. The objective of this paper is to show that in many such cases interfacial equilibrium has not been achieved and that incomplete wet ting is a major factor in establishing the performance. This proposal is based on the adhesive performance of many polymer-substrate com binations observed as a function of temperature [4]. Discussion Wetting may be thought of as the process of achieving molecular contact. The extent of wetting may be defined as the number of molec ular contacts between the two phases comprising the system relative to 180

In Contact Angle, Wettability, and Adhesion; Fowkes, F.; Advances in Chemistry; American Chemical Society: Washington, DC, 1964.


7 7.

HUNTSBERGER

Wetting

and

181

Adhesion

that exhibited when wetting is complete. This definition, in contradis tinction to one which equates wetting with spreading, is preferable for discussion of relationships between wetting and adhesion. Equilibrium contact angles are functions of the surface free ener gies of the solid substrate and the liquid in contact with it and of the free energy of the interface between the two phases. Much useful i n formation can be obtained through studies of contact angles. This equilibrium, however, represents the extent to which a liquid spreads over the substrate and not the extent of wetting at the interface. In practice the fluid adhesives are spread over the substrates and constrained in some way so that the problem is to determine whether the surface free energies and interfacial energy of the phases in ques tion will lead to complete wetting. A fluid adhesive spread over a " r e a l " (nonplanar) substrate is shown schematically in Figure 1. The system is depicted in a state in which the solid is not completely wetted (upper) and the wetted state (lower).

Figure 1. Wetting of solid substrate by fluid adhesive Upper. Substrate incompletely wetted Lower. Substrate completely wetted

Johnson and Dettre [5] have investigated the thermodynamics of wetting for specific nonplanar surfaces and have shown that both stable and metastable equilibria can be encountered in systems exhibiting i n complete wetting. By considering the change in surface free energy associated with the change from any given nonwetted state to the wetted state for a single asperity, it is easy to find a criterion for wetting. Energy changes in other parts of the system will be negligible if the volume of the asperity or not wetted void is small compared to the total volume of the adhesive layer and will not invalidate this treatment. Neglecting gravitational effects, adsorption, and any thermal ef fects associated with the morphology of the adhesive, the change in free energy accompanying the wetting of the substrate in any single asperity is given by: AF =

fi F sv

SL

-

[fl

s v

F

s v

+

A

L V

F

L V

]

(1)

where Ω ^ and A are the integrated or actual area of the solid-vapor interface and the area of the liquid-vapor interface, respectively, in L V


ADVANCES IN CHEMISTRY SERIES

182

the nonwetted state. and F ^ are the surface free energies of the solid and liquid phases, respectively, in equilibrium with the saturated vapor phase. F ^ is the free energy of the solid-liquid interface. Under the above assumptions the Young-Dupre' equation can be written in terms of surface free energies: y

F

SV

-

F

SL

"

F

LV

C

0

S

( )

Θ

2

substituting this value for F ^ - F ^ in Equation 1 gives: AF = - F

L V

[l + ( Ω ^ / Α ^ )

cosfl]

(3)

Equation 3 shows that at equilibrium complete wetting is expected unless ( Ω ^ / Α ^ ) cos θ < -1. It is important to note that this criterion establishes whether the minimum surface free energy is exhibited by the wetted or the particular given nonwetted state. Some surface geom etries could be found which during the wetting process would lead to stable equilibria exhibiting surface free energies lower than either the initial or wetted states given above [5]. F o r most practical adhesive systems, the adhesive will exhibit equilibrium contact angles with the substrate of less than 90°. Cos θ will be positive; and thermodynamic equilibrium will correspond to a completely wetted state.


1

Experimental

Procedures and Results

The adhesive performance of poly(n-butyl methacrylate) adhered to cold-rolled steel substrates was determined as a function of tem perature. The polymer exhibited an inherent viscosity of 0.57 (0.5% in chloroform). F i l m s were applied to the steel substrates by casting from 40% solid solution in reagent grade toluene. To obviate stretch ing and tearing during testing, 2-mil-thick glass fabric was embedded within the polymer by casting two coats, pressing the glass fabric into the second coat immediately after casting the film. The films were applied using an applicator with a 20-mil clearance. The resulting structure was 9 mils thick with approximately 7 mils of polymer below the fabric. The cold-rolled steel was abraded with 600A silicon c a r bide abrasive and rinsed in acetone. The peel strength was determined using the apparatus shown in Figure 2. The 1-inch-wide substrate panels were placed in Teflon TFE-fluorocarbon resin lined tracks. The free end of the film was passed around the roller and attached to a line which extended through a small tubular opening out of the oven and around a ball-bearing pulley to the loading container. Tests were carried out by adding lead shot slowly to the container until peeling proceeded at a rate of 2 to 3 mm. per minute. A s the films were peeled, the substrate panels slid in the tracks under a sec ond small retaining roller. The peel angle was set by the nature of the system and was observed to be somewhat less than 90°. The precision of this test is a function of the temperature and of the strain geometry. It can be defined most conveniently by grouping the data for cohesive failures and for apparent interfacial failure sepa rately. The coefficient of variation for the mean of duplicate tests was ±4.3% when failure was cohesive and ± 9.0% when failure appeared to


7 7.

HUNTSBERGER

Wetting


OVEN

and

183

Adhesion

CHAMBER

Figure 2. Schematic diagram of appa ratus for measuring peel strength at various temperatures occur at the interface. The data are plotted as the log of the load r e quired for peeling vs. reciprocal absolute temperature. The points are based on the mean values of the duplicate tests. Two sets of samples were prepared. After 3 hours had been a l lowed for solvent evaporation, the adhesive bonds were formed by hold ing one set for an hour at 100°C. and the second for an hour at 150°C. The results are shown in Figure 3. I20°C

? "

CO

3.4 3.2

M d © ζ ο

3.0 2.8 0.0025

0.0027 0.0029 l / T (RECIPROCAL ABSOLUTE

0.0031 TEMPERATURE)

Figure 3. Force required to peel poly(n-butyl methacrylate) from steel vs. reciprocal absolute temperature




184

At temperatures above ~72°C. the performance was identical for the two samples. Failure occurred cohesively within the polymer. At temperatures below - 7 2 ° C . the peel strength of the sample bonded at 150°C. continued to increase with decreasing temperature but at a r e duced rate, and the locus of failure shifted to or close to the interface. The sample bonded at 1 0 0 ° C , however, exhibited decreasing peel strength with decreasing temperature, reaching a minimum at ~50°C. Below 50°C. the peel strength again increased. Below 72°C. the failure appeared to occur at the interface. This behavior appears to be satisfactorily explained by the follow ing hypothesis: The two major factors influencing the performance are the viscoelastic response of the polymer and the extent of interfacial contact. The difference in the behavior of the two samples is attributed to the fact that the sample bonded at 150°C. had reached or at least very closely approached interfacial equilibrium and had achieved nearly maximum interfacial contact or "wetting." The sample bonded at 100°C. did not approach equilibrium, and discontinuities existed in the interface of such magnitude that stress concentration occurred at their edges when the effective strain rate of the test exceeded the relaxation rate of the polymer. F o r this test and with these particular samples the effective strain rate and relaxation rate were equal at - 7 2 ° C . Above this temperature cohesive failure occurred through polymer r e laxation. At temperatures below 72°C. stress concentration arose and increased with decreasing temperature as the relaxation rate of the polymer decreased and its modulus increased. Failure occurred when the stress at the "microedges"of the discontinuities in the sample p r e pared at 100°C. exceeded either the interfacial bond strength or the cohesive strength of the polymer, depending on the locus of failure. F o r the sample prepared at 150°C. concentration of normal stresses occurred only at the sample edges; hence only a small deviation in the performance was observed at temperatures below 72°. The stresses were greatest at or very near the interface, however, because of the nature of the deformation of the sample during testing. This resulted in the shift of the locus of failure. The preceding hypothesis is substantiated by data for many sys tems obtained through peeling tests and tests involving cleavage of lap joints. A particularly interesting example is provided by the behavior of poly(n-butyl methacrylate) bonded to a substrate exhibiting a low surface energy. The low energy substrate was achieved by coating a steel panel with 52% soybean oil alkyd resin with an acid number of 6. The resin was cured by baking for 0.5 hour at 200°C. The surface energy was appraised using the technique of Fox and Zisman for evalu ating y . This gave a value of y -31 dynes per cm. Three sets of samples were prepared using bonding temperatures of 120°, 150°, and 180°C. Bonding time was 1 hour. The data are shown in Figure 4. The point of greatest interest is the similarity of the performance as compared with that exhibited on the high energy steel substrates. The important difference is that higher temperatures were required at equal bonding times to reach equivalent performance. This is what would be expected if the bonding were a "wetting" process. Another interesting example is given by the behavior of 0.5 x 0.5 inch lap joints comprising 0.25 χ 0.5 x 5 inch steel bars bonded with c

c


7 7.

HUNTSBERGER

Wetting e

120 C

e

100 C

and

185

Adhesion

80 °C

e

40 C

e

60 C e

SAMPLE 1 - BAKED 1 hr. AT 180 C SAMPLE 2 - BAKED 1 hr. AT 150 C SAMPLE 3- BAKED 1 hr. AT 120 C 0

e

4.0

i s CO

Ul - I X UJ Ο UJ

3.8 3.6

< Q. 3.4


8

-COHESIVE FAILURE • APPARENTLY INTERFACIAL FAILURE

J-o

,
2.8 0.0025

0.0027 0.0029 l / T (RECIPROCAL ABSOLUTE

0.0031 TEMPERATURE)

Figure 4. Force required to peel poly(n-butyl methacrylate) from an alkyd resin vs. reciprocal abso lute temperature poly(methyl methacrylate) exhibiting a viscosity average molecular weight of ~90,000 and plasticized with various amounts of benzyl butyl phthalate. The samples were bonded by holding under a load of 10 pounds for 3 hours at 150°C. After this time the samples were cooled slowly and tested in situ at various temperatures as the cooling pro ceeded. This avoided a history of thermal cycling. The apparatus is shown diagrammatic ally in Figure 5. (The sample-holding device has been eliminated for clarity.) The tests were carried out by adding lead shot to a container attached to the end of the test line at a rate of 100 grams per second. The data are shown in Figure 6. The solid lines again indicate cohesive failure, and the dashed lines indicate apparent interfacial separation. The shift of the position of the maximum to lower temperatures with increasing plasticizer content is consistent with the interpretation given above. The smaller decrease at temperatures below the maxima with increasing plasticizer content may reflect increasing proximity to equilibrium, since all samples were prepared by holding at 150°C. for 3 hours. These three sets of experiments all appear to be in complete har mony with the proposed explanation. A satisfactory independent deter mination of the extent of interfacial contact would be very desirable, however, in order to establish the validity of the hypothesis. The physical dimensions of the interfacial discontinuities appear to be such that they cannot be detected by visible optics, nor have first attempts to show their existence by capacitance measurements been fruitful. Thus far it has been necessary to rely on data which support the proposition but do not provide unequivocal evidence that incomplete "wetting" is primarily responsible for the observed performance. An example is the "activation energy" of the bonding process. This has




186

w Figure 5. Schematic diagram of appa ratus for measuring force required to cleave lap joints at various tem peratures L . Line for application of load for bond f o r mation. T . Line for application of test load e

120 C "I

e

80 C

100* ι—

e

40 C

e

60 C 1

1 - PMMA 2» PMMA/BBP-95/5 3- PMMA/BBP» 90/10 4- PMMA/BBP* 80/20 5» PMMA/BBP» 70/30 0.0025

0.0027

0.0029

0.0031

l / T (RECIPROCAL ABSOLUTE TEMPERATURE) Figure 6. Force required to cleave 0.5 x 0.5 inch lap joints comprising 0.25-inch thick steel rods bonded with plasticized polyfmethyl methacrylate) vs. reciprocal absolute temperature



7 7.

HUNTSBERGER

Wetting

and

187

Adhesion

been determined for the bonding of p o l y v i n y l acetate) to steel, giving a value of 42 kcal. This appears to be in fair agreement with data r e ported for the activation energy for flow (39 kcal.). A similar obser vation has been made recently by Gusman [3], If the wetting process involves viscous flow, at some thickness level the thickness of the polymer film should exert a detectable i n f l u ence on the rate of bonding. The rate of leveling of surface scratches in lacquer films, for example, has been shown to increase with i n creasing film thickness. By analogy the rate of bonding at the liquidsolid interface might be expected to decrease with decreasing film thickness. Such an effect was observed with both poly(n-butyl methacrylate) and polyethylene films when they were 50 to 1000 A . thick. This lends strong support to the concept that wetting rather than chem ical interaction is frequently involved in the bonding process. If these hypotheses are correct, one of the major problems is to establish the factors which restrict or limit the interfacial contact. Consider first the case of fluid low molecular weight polymers of moderate viscosity which may be applied without solvents and " c u r e d " in situ to give useful films or adhesives. If no external pressure is applied, the only driving force leading to wetting is the capillary pres sure. Zisman [6] has shown that a maximum capillary pressure exists for capillary pores in a given material when

?LV

=

1 2b

1 2 Ύο

+

Constant b in Equation 4 is a measure of the interaction between the solid and liquid phases when cos Θ is approximately a linear function of y , cos 0 = 1 + b(y - y ) , and y is the " c r i t i c a l surface tension" of wetting of the solid [l]?" y is related to the surface free energy of the solid: L V

c

L V

c

c

y

c

= *

2

f

w

(5)

where φ is a measure of the departure from regularity in the interfacial interaction between the solid and the liquid [2]. The maximum capillary pressure is given by:

ϊ) ( H ) where r and r are the principal radii of curvature of the liquid-vapor interface in the nonwetted regions of the solid-liquid interface. For organic substrates the first term will generally be about 20 to 70 dynes per cm. The second indicates a profound influence for sur face topography. This term could have a large positive value for small porelike asperities but very small or even negative values for some configurations. This may explain in part the improved adhesion often observed for sand-blasted or etched surfaces in contradistinction to sanded or polished surfaces. Certain topographical configurations may lead to metastable mechanical equilibria which would halt the wetting process. x

2




188

The nonwetted portions of the interface comprise small closed gas- or vapor-filled volumes bounded by the substrate and the adhe sive. If these volumes are of such size that they contain a sufficiently large number of molecules in the vapor phase to exert nearly normal pressures, these will act against the capillary pressures and diminish the rate of wetting. It is conceivable that diffusion of these molecules into the adhesive layer could be rate-limiting under certain circum stances. Also in general the resistance to viscous flow will increase at a faster rate than the capillary pressures increase as the pore or chan nel dimensions become smaller. This is of real significance in the case of hot-melt adhesives, which are frequently applied at relatively high viscosity and remain fluid for only a brief time. For polymers applied from solution there is a possibility that the solvent may be selectively adsorbed. If this were the case, intimate contact between the polymer and substrate could be achieved only after desorption of the solvent. If desorption occurred when the solvent con centration was relatively small, the viscosity of the solution could be very high, precluding wetting within the time the solution exhibited sensible fluid characteristics. If coacervation were to occur in the solution in the vicinity of the solution-substrate interface, the fluid phase containing relatively little polymer would wet and fill the surface asperities. After evaporation of the solvent the voids would remain unfilled. Conclusions Virtually all practical systems should exhibit complete wetting at equilibrium. In practice, however, interfacial equilibrium is frequently not achieved. Under these circumstances high stress concentration at the interfacial discontinuities leads to poor performance. The stress concentration is determined by the viscoelastic response of the adhe sive and the time-temperature characteristics of the test used to as sess the performance. Some factors which may retard the wetting and preclude attainment of equilibrium are a combination of low capillary pressures and high fluid viscosities, metastable equilibria, selective adsorption, and co acervation. Literature Cited (1) (2) (3) (4) (5) (6)

Fox, H. W., Zisman, W. Α., J. Colloid Sci. 7, 428 (1952). Good, R. J., Girifalco, L. Α., J. Phys. Chem. 64, 561 (1960). Gusman, S.. Offic. Dig. Federation Soc. Paint Technol. 34, 884 (1962). Huntsberger, J . R., J. Polymer Sci. 1a, 2241 (1963). Johnson, R. E., Jr., Dettre, R. H., Advan. Chem. Ser., No. 43, 112 (1963). Zisman, W. Α., General Motors Symposium on Adhesion and Cohesion, July 24-25, 1961.

Received March 14, 1963.


Contact Angle, Wettability, and Adhesion

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