INTRODUCTION
Recently there has been growing interest in and actual need for icing-preventive materials. Though the actual application of such materials is limited to some restricted regions and periods in the field of navigation, aviation and others, the problem has been serious when accidents have taken place involving human life.
This work relates to ice adhesion to polymeric substrates and consists of the following two parts:
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1) Fundamental studies of ice adhesion
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2) Development of ice repellent coating.
FUNDAMENTAL STUDIES OF ICE ADHESION
There are numerous papers on the thermodynamic interactions between water and substrates; however the energetic analysis by the measurement of icing contact angle has not yet been made. This is possibly due to difficulty of assessment of the surface-free energy of ice and associated problems. In the present work such problems have been solved and a method by which ice adhesion can be treated thermodynamically has been established.
Thermodynamic consideration of ice adhesion
The specific work of adhesion W-πe in the air can be represented by the correct form of the Young-Diipré relation (Dupré 1869; Fox and Zisman 1950) modified by Bangham and Razouk (1937).
where yi is surface free energy of ice, θ is the contact angle of ice/substrate and πe is the reduction in surface energy of the solid resulting from adsorption of vapor from liquid being measured.
On the other hand, when the water droplet on a substrate in liquid paraffin cools into ice πe will be eliminated. Energetic equilibrium condition is represented by
where ysl ysi - yilare interfacial free energy of substrate/paraffin, substrate/ice and ice/paraffin,
respectively. And θ1 is the contact angle of ice/substrate in paraffin. There are well known equations on the work of adhesion W, and interfacial free energy,
where ys, yi, y1 [ and yds1 are surface free energy of substrate, ice, paraffin and dispersion component of ice, respectively. And θ2, is contact angle of paraffin on a substrate. From the equations 2, 3, 4 and 5 W is expressed by
Using Equation 6, work of adhesion W can be determined by measurement of contact angles θ1 of ice/substrate in paraffin and θ2, of paraffin on a substrate in the air and from surface free energy of ice and paraffin.
Calculation of surface free energy of ice
Fowkes (1964) determined dispersion and hydrogen bond components of water at 20°C 21.8 and 51.0 mJ/m2, respectively. Surface and interfacial free energy of ice and ice/water at 0°C are determined as 109.0 and 33.0 mJ/m2 by Hobbs (1974).
On the other hand, the heat of vaporization of water at 0°C is 44.9 kJ/mol and latent heat of freezing 6.0 kJ/mol. These interna! energies must correspond to the surface contribution of 75.6 and 33.0 mJ/m2, respectively. Dispersion energy can be expressed (Tabor 1978) by the work required to separate the adhered parallel plates infinitely.
where E is dispersion energy, f is attractive force per unit area, ho is distance between molecules, A is Hamaker constant. This energy is equivalent to cohesive energy 2y and ho will be proportional to one-third power of molar volume. Then y is proportional to two-third power of molar volume Vm. By the use of this relationship dispersion component of water and ice at any temperature can be calculated. Assumed all heat of deicing involved in the contribution from dissociated hydrogen-bond of ice, dispersion and hydrogen bond energy at 0°C will be 20.59 and 88.41 mJ/m2, respectively. Moreover, vaporization energy at O K is 56.1 kJ/mol. The values at an arbitary temperature can be calculated by interpolation. These relationships are shown in Figure I. On the other hand, surface free energy of liquid paraffin {Witco chemical, Kaydol) was determined by Wilhelmy-Plate method (Klungness 1981) at 20°C and pendant drop method at 20°C and -15°C.
Fig. 1. Dispersion and hydrogen bond components of surface free energy of water and ice.
EXPERIMENTAL
Materials
Polymeric materials used in this investigation are listed in Table I. Resins No 1 to 13 were coated by solidification from solution on 2 mm thick aluminum plates cut into 70x90 mm for the measurement of shear strength, and on the 10 mm thick stainless steel plates of the same size for the tensile measurement. Plastic plates of No 14 to 22 were cut into test panels in the same size described above and were adhered to 10 mm thick stainless steel plates for the two kinds of measurements.
Apparatus and test methods
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a) For the measurement of contact angle of icing we used a goniometer (Kyowa Kaimenkagaku) consisting essentially of a contact angle meter, TV monitoring system and circulating bath for the temperature regulation. Test pieces were made in the size of 25x8x3 mm. Resinous test pieces are used without treatment, whereas plastic test pieces are cleaned 3 times with distilled deionized water and finally with methanol. Test piece prepared is set in a pyrex-cell (30x20x14 mm). Then contact angles of water and ice formed on the polymeric substrates are measured at 20 C and -15°C in the air and in paraffin, respectively. For the droplet, distilled deionized water is used and its volume is 50μl.
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b) For the measurement of adhesive strength of ice we developed a new programming test apparatus for the measurement of the force of ice adhesion with good accuracy and reproducibility. The apparatus consists essentially of a temperature regulated chamber, in which specimen panels are mounted firmly on a base turn-table, a driving device which generates “pull and push” force connected to a load cell, a process controller and a recorder. Stress cycles, cooling-heating schedule can be programmed freely.
Measurement of shear strength
The metal ring with inside section area 5 cm2, height 1.5 cm is set on polymer surface of test plates and precooled for 90 minutes at predetermined temperature, then 2 ml of 5 C distilled-deionized water is poured in the ring. After keeping for 3 hours at that temperature shear strength of ice adhesion is measured.
Measurement of tensile strength
The apparatus consists of an attachment of stainless steel with 10 cm2 cross sectional area. 30μ1 of distilled-deionized water is sandwiched between the specimen and a polymer substrate so as to form an ultra-thin layer (30μτη) of water. The whole apparatus is then refrigerated to predetermined temperature for 3 hours at that temperature. Then tensile strength is measured.
The work of adhesion W- πe, and W for different polymers can be seen in Figure 2. Figures 3 and 4 show the relationships of theoretical vs. observed ice adhesion by shear method, respectively.
Physical properties and ice adhesion
In order to investigate the susceptibility of physical properties of polymers to adhesive strength of ice visco-elastic behaviour particularly below icing temperature’ on some polymers, using TBA (Tortional Braid Analysis Toray Rigaku) was studied. Further, the temperature dependence of strain in the ice/polymer system, in which ice adhered the polymer surface, was studied with a DSA (Dynamic Strain Amplifier, Kyowa Electronic Institute).
RESULTS AND DISCUSSION
The values obtained for contact angle of ice in air were in most cases smaller than that of supercooled water, whereas contact angles in paraffin were not so characteristic. πe must be obtained from the difference W - (W - πe). As can be seen, πe values are negative with a few exceptions, although from a theoretical viewpoint, these must take positive values. A possible explanation for this is the very strong, momentary interaction of supercooled water molecules with the adsorbed ones on icing. It has been revealed, that the value for πe ignored by Landy and Freiberger (1967) can be evaluated by this method.
Table 1. POLYMERIC MATERIALS USED IN THE INVESTIGATION.
Fig. 2. Work of adhesion between polymers and ice determined thermodynamically at -15°C.
Fig. 3. Relationship of work of adhesion (W - πe) vs observed adhesive strength of ice in shear at -15°C.
Generally, it can be said that polymers having lower values for W result lower ice adhesion to that. By the tensile method at -15 C, cohesive fracture accurred in most cases. The measured values are thought to be valid only when the measurement is made at -10°C and with polymers of low surface energy. Temperature dependence of ice adhesion was studied. The gradient obtained in the group of acrylic polymers is much larger than that of energetic values calculated, whereas with fluoro-, silicone- and oleflnic resins, temperature dependency is not so conspicuous.
As can be seen in Figure 5 and Table 2, the susceptibility of polymer to icing depends on its visco-elastic behaviour in a low temperature zone as well as on the properties of surface. It is difficult to differentiate the contribution of surface energy and visco-elastic properties to ice-mitigative effects, because low energy polymers generally have low glass-transition temperatures. However, it is clear, that higher rigidity or higher glass-transition temperature such as IBA, TFE and solid paraffin (PF) at low temperature affects to reduce ice adhesion negatively. DSA investigation (Figure 6) showed that the magnitude of contraction strain of the film in the vicinity of icing interface is proportional to tensile strength of ice adhesion. From the results mentioned above, the physical characteristics of polymers involved in molecular mobility can be a necessary condition to reduce ice adhesion.
DEVELOPMENT OF ICE REPELLENT COATING
Through the fundamental experiments, some relationships of energetic and physical properties of polymers with ice adhesion were revealed. From the results, it is presumed that any polymeric material with an ordinary structure cannot provide the performance to prevent from ice adhesion completely, even if the polymer characteristic found are set up optimum. As the results of investigations from various aspects, a composite material, to which ice adhesion can be remarkably prevented, was found.
Fig. 4. Relationship of work of adhesion (W) vs observed adhesive strength of ice in shear at -15 C.
Fig. 5. Mechanical damping and relative rigidity of polymers at low temperatures.
On the relationship of theoretical vs, observed interaction by shear fracture, considerable linearity was obtained, although so many other properties of polymer were not taken into consideration. The adhesive strength of
Table 2. CORRELATION OF WORK OF ADHESION (W), VISCO-ELASTIC PROPERTY (Tg) VS ICE ADHESION.
Fig. 6. Temperature dependence of strain in ice/polymer system and tensile strength of ice adhesion.
Composition and mechanism for prevention from ice adhesion
Basic studies of mechanism of icing from viewpoints of thermodynamic and physical factors were made. In the course of numerous attempts, it has been found that ice adhesion can be almost completely prevented by the synergetic effect resulting from the combined use of an organopolysiloxane resin and alkali metal compound. The trial product of this coating material is tentatively named IRC. The film made from this composite materials has characteristic surface properties, and shows amphibious nature; hydrophobic and hydrophilic.
Fig. 7. Schematic representation of bound water hydrated on a lithium ion.
The organopolysiloxane resin has a chain of hydrocarbon atoms arranged on its surface and thus is low in surface energy resulting hydrophobicity. This resin contains a small amount of a polar ingredients prone to form hydrogen bonds. This is the reason for the formation of the polymers considerably amenable to icing. Alkali metal compound, especially containing lithium atoms, shows strong hydrophilicity. The lithium ion has a small Pauling radius (0.06 nm) so that the hydration energy is great as 5.23 x 102 kJ mol-1 and acts therefore as a hydrogen bond breaker. It is confirmed by Hiraoka and Yokoyama (1982), that the four oxygen atoms from water molecules and two oxygen atoras contained in a carboxylic residue associate together to form the structure of octahedron coordination. The water molecules thus hydrated on lithium ion are said the bound water or non freezing water. Schematic representation of the structure is shown in Figure 7. Around the bound water, some layers of water molecules so-called the restrained water are arranged, and surrounded by the free water. Each three types of water will behave differently at icing temperature. It is assumed that structural and energetic differences thereof and synergetic effect from silicone matrix result in preventing ice adhesion. DSC analysis of this film showed that the water absorbed in the film up to 8% does not have a definite freezing point, and at the 15% the water showed a broader freezing range, namely 0~-28°C. furthermore, the relaxation times of the equilibrium water absorbed in freezed film at -20 °C were studied by Pulsed NMR. It was clear that the structure of water molecules are in the state “restrained”. The adhesive strength of ice to the film in comparison with other materials of low ice adhesion is shown in Figure 8. Monitoring tests are under way in many fields of application.
