Contact Angle, Wettability, and Adhesion

22 The Wettability of Road Aggregates with Doped Bituminous Binders

Downloaded by GEORGETOWN UNIV on August 27, 2015 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0043.ch022

R. P. DRON Laboratoire Central des Ponts et Chaussées 58 Boulevard Lefebvre Paris, France The ability to coat wet aggregates is connected with immersion adhesion tension and antistripping effect is connected with emersion adhesion tension. These values are measured by Guastalla's wetting balance for the system water-kerosine-mineral slide. Increasing amounts of the studied dope are dissolved in kerosine. Two thresholds of concentration were found. The lower one may be related to the antistripping effect; the higher may be related to the coating of the wet aggregate.

While analyzing the adhesive qualities of binders on aggregates, several authors have pointed out the leading part played by wetting. Duriez [1] underlines the necessity of considering the ternary system mineral-binder-water to explain the affinity for coating wet materials on the one hand and the stripping of coated materials by water on the other hand. He emphasizes the role played by dopes as modifiers of the absolute value of the adhesion tensions of water and binder on the minerals, which are the motivating factors behind the development of coating, the viscosity of the binder and the roughness of the stone acting as a brake. Hallberg [4] has shown that, from an experimental point of view, it is of interest to replace the binders by liquid hydrocarbons; this e l i m i nates viscosity, which is an interfering factor. The qualitative sense is not seriously altered, as the chemical properties of the hydrocarbons, which control the wetting phenomenon, are very closely related to the properties of the bituminous binders. Nevertheless, the experimental method which Hallberg suggests, based on measuring the speed of capillary ascension of the hydrocarbon in sand, does not eliminate the roughness of the mineral. It is of interest to know the adhesion tensions by direct measurement. According to whether or not the forces put in action by the 310

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


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surface phenomena are able to overcome the forces of viscosity, evolution or stabilization of the coating state will take place. These surface actions appear only when water and binder coexist on the mineral and it is necessary, then, to study the phenomenon at the line of contact between the three phases. Such a knowledge is particularly valuable for the study of the additives known as dopes. The action of the dopes is precisely that of modifying, by surface adsorption, not only the absolute values of adhesion tensions and of interfacial tensions, but also the signs of these forces, which are indicated by a change of the contact angles. Inspired by Hallberg's method, we have taken up the problem of r e placing the binder by a liquid hydrocarbon. However, instead of using sand, we worked with slides of polished minerals in order to eliminate the roughness factor. A s Duriez indicated, it is not necessary to study a great variety of minerals; it is enough to establish a distinction among the acid minerals, composed of silicates, and basic materials (alkaline earth carbonates). Our work proved that there is no essential difference within each of these two categories. The hydrocarbon chosen is kerosine, which, like asphalt, is of petroleum origin and is sufficiently fluid without being too volatile. We have been able to define the difference in behavior of the mineral-hydrocarbon-water system according to whether the mineral is initially wetted by water or by hydrocarbon, by utilizing J . Guastalla's wetting balance which permits one to measure successively the immersion, and emersion, adhesion tensions. Experimental

Technique

Guastalla Wetting Balance. This instrument [2,3] allows one to measure the force exerted on a vertical slide (2.5 x 2 cm.) when it is partially immersed in a liquid o r , as in the case with which we are concerned, when it is placed perpendicular to the interface of two l i q uids, the upper part of the slide being in the lighter liquid and the lower part in the heavier one. The slide is hung on a lever which exerts a torsion on the torsion wire and the distortion is optically amplified by a m i r r o r system (Figure 1). The reading is done on a graduated scale in dynes per centimeter. The entire torsion balance is mobile in the vertical sense, which allows displacement of the slide either upward or downward, in order to immerse or withdraw it progressively. This shifting is done by means of a drum, where each turn corresponds to a change of level of 1 mm. Technique of Measurement. A beaker containing water and kerosine is placed under the torsion balance, each layer having a thickness of about 2.5 c m . The slide being studied is hung on the lever and completely immersed in the water layer before the layer of kerosine is floated on top of the water. By turning a knurled knob, the wire is twisted so that the spot indicates zero. The drum is then turned to raise the slide until its upper part comes in contact with the interface. The position of the drum is used as the point of departure for measurements of vertical displacement.


312

LEVER -


KEROSINE

WATER —

Figure 1. Guastalla's wetting balance Tracing of Wetting Cycle The slide is raised progressively, millimeter by millimeter, and the force exerted on it is read on the scale at each position. Equilibrium is obtained after 1 minute. The force increases progressively for the first few millimeters and then stabilizes. This stable value corresponds to the immersion adhesion tension. The slide is then progressively lowered; the reading is stabilized at the end of several turns of the drum. This second reading c o r r e sponds to the emersion adhesion tension. If the values measured, while the slide is being raised and lowered, are taken into consideration in relation to the number of turns of the drum (in other words, the portion of the slide immersed in the kerosine),a parallelogram is obtained,two sides being horizontal when the slide is extremely thin. Because of the different buoyancies in the two liquids, a slight slope is noticeable and the exact values of the immersion and emersion tensions are obtained by extrapolation to zero (Figure 2).

Figure 2.

Tracing of wetting cycle



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The slide is then replaced by a platinum stirrup (2.5 cm. long) in order to determine the interfacial tension. The tearing free method is employed, as usual, and a direct reading is obtained on this instrument. The relation between the adhesion and the surface tensions gives the cosine of the contact angle. Preparation of Slides. A 2.5 x 2 c m . slide, about 0.5 mm. thick, is sawed and ground from a sound and homogeneous part of the stone being studied. It is then polished with finer and finer abrasives and the operation is finished with alumina obtained after 5 hours of sedimentation. Then, using a diamond point, a small hole is pierced, by scratching, in the middle of the upper edge of the slide, so that it may be hung on the balance. The slides are cleaned before each experiment by placing them for one hour in methylene chloride vapor, inside a flask topped by a reflux cooler. We verified that this process is sufficient to obtain reproducible measurements (± 1 dyne per cm.). A Study of Mineral Wetting in Presence of Dopes Systems Studied. We have made slides of various materials and studied the variations of immersion and emersion adhesion tensions by adding Dinoram S,an amine currently utilized in France for doping road materials and similar to the American Duomeen T , which has an alkyl propylenediamine base. The water-kerosine interface is always used and the amine is dissolved in the kerosine phase. The action of amine hydrochlorate or amine acetate can also be studied by dissolving them in water. The slides were cut from quartz crystal and from porphyry and basalt. It was not possible to make slides from calcite crystal because of its extreme cleavability. Figures 3 and 4 show the immersion and emersion tensions on various materials vs. the logarithm of the dope concentration. The dashed curve shows the interfacial tension. Examination of Results. Interfacial Tension. The interfacial tension of water-kerosine is equal to 40 dynes per c m . in the absence of dope. It is not affected by the presence of dope unless its concentration in kerosine increases beyond a certain value, which is approximately 1 0 " gram per m l . It then decreases linearly vs. the logarithm of the concentration and attains very low values which are not measurable when the concentration reaches 1 0 " gram per m l . The slope of the linear part is: 7

2

d r / d log c = 12 dynes per c m . The immersion adhesion tension is negative and nearly equal in absolute value to the interfacial tension, as long as the dope concentration remains low. It deviates from this value when the concentration attains 10~ gram per m l . The interesting fact is the cancelling out and the change of sign of the adhesion tension for a certain threshold of concentration which changes with the nature of the solid. At high concentration the immersion adhesion tension is equal in value and sign to the interfacial tension. The curve of immersion adhesion tension concentration ultimately joins the curve of interfacial tension and this means that the contact 6


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Figure 3. Immersion adhesion tensions 1. 2. 3. 4.

Figure 4.

Basalt Quartz Porphyry A Porphyry Β

Emersion

adhesion tensions

angle becomes nil and the wetting by hydrocarbon is perfect. The emer sion adhesion tension varies greatly with the materials in the absence of dope. It is negative in the case of quartz, positive for other materials. It is affected by dope as soon as the concentration attains 10 " gram per m l . and becomes equal to the interfacial tension when the concentration is equal to 10 " gram per m l . The results are appreciably different when working with amine salts, introduced in the equeous phase. 7

5


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Aggregates

With amine acetate at a higher concentration (10" gram per ml.) the interfacial tension is nearly 5 dynes per c m . , but the adhesion tensions are close to zero, so that the contact angles are equal to 90°. With amine hydrochlorate, the problem i s still more paradoxical. The adhesion tensions are negative and their absolute values may, in a high dope concentration, be equal to the value of the interfacial tension, but of opposite sign. The solid i s , therefore, perfectly wetted by water. 2


Interpretation

of Results

The technological problems relative to the competing of water and binder on the surface of the stone are of two categories. Problem 1. A wetted stone is to be coated with a binder. This problem arises at the time the macadam is made. Water is to be displaced by the binder on the surface of the mineral. This process is equivalent to the immersion of the slide in thekerosine phase. We must consider, therefore, the immersion adhesion tension. The immersion tensions of all the materials studied are negative in the absence of dope. Therefore, it is not possible to displace a film of water by the binder in the absence of surface active agents. When an amine is dissolved in the hydrocarbon phase, a threshold of concentration exists, beyond which the adhesion tension becomes positive. This threshold is characteristic of the efficacy of the dope employed on the mineral. It is equal to the concentration of the dope required for the binder to displace the water off the mineral surface. When the dope is in its hydrochlorate state and dissolved in the water phase, it acts only on the interfacial tension; its role seems unfavorable from the wetting point of view. A film of water is drawn along the mineral when it is immersed in kerosine, probably because of the adsorption of whole micelles and no longer of isolated molecules as Ter-Minassian found on glass slides [6]. Thus, the outer part of the adsorption coating is hydrophilic instead of lipophilic. When acetate is utilized, the contact angle is near 9 0 ° . The additive acts only to lower the tension and not as a wetting agent. Problem 2. A different problem appears when the material is set in place on the road and we want to avoid displacement of the binder by water. It is a matter of protecting the structure, if, for example, the laying is followed by rain. This is equivalent to the emersion process and the force of displacement is then the emersion adhesion tension. The emersion adhesion tensions at the lower concentration of dope indicate different tendencies to spontaneous stripping according to the minerals. The spontaneous stripping can occur only on quartz, for which the adhesion tension is negative. However, although positive, the emersion adhesion tensions are very low (practically nil in the case of porphyry). Because of the hysteresis (difference between immersion and emersion adhesion tensions) under the effect of outside energy—mechanical action, for example—the binder may be irreversibly replaced by water, and the stripping will continue, leading to the destruction of the structure. For the quartz, a threshold of concentration of the dope exists, above which the emersion adhesion tension becomes positive and the spontaneous stripping can no longer take place; its value is near 5.10" gram per m l . 7


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316 Conclusions


Measurements of adhesion tensions on polished mineral slides by the Guastalla wetting balance point out certain aspects of the physical mechanism of the wetting of aggregates with doped hydrocarbons in the presence of water. This technique permits one to determine whether the aggregates are initially wetted by water or by hydrocarbons. The results show that water-wetted materials can be made receptive to coating by adding a certain quantity of amine, and that spontaneous stripping can occur only on very acid minerals and may be prevented by adding a smaller quantity of amine. Literature Cited (1) Duriez, M., Construction, special issue (December 1958). (2) Guastalla, J., J. Colloid Sci. 11, 623-36 (1956). (3) Guastalla, J., "Proceedings of Second International Congress on Surface Activity," p. 143, Butterworths, London, 1957. (4) Hallberg, Sten, Meddelande 78, Statens Väginstitut, Stockholm, private communication, 1950. (5) Rosano, H. L., Weill, M., "Memorial des Services Chimiques de l'Etat," Vol. 37, part 3, 1952. (6) Ter Minassian-Saraga, L., 5th Colloquium, "Theorie et Pratique de l'Utilisation des Agents de Surface," Paris, June 1959. Received April 3, 1963.


Contact Angle, Wettability, and Adhesion

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