Contact Angle of Epoxy Resin Measured by Capillary Impregnation and the Wilhelmy Technique

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1 Contact Angle of Epoxy Resin Measured by Capillary Impregnation and the Wilhelmy Technique Min Li, Zuo Guang Zhang* and Zhi Jie Sun School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing , China Received: 16 May 2005 Accepted: 28 July 2005 SUMMARY Good wetting plays an important role in improving the degree of adhesion at interfaces between fibres and matrices in composites. The contact angle of an epoxy resin, as a widely used polymeric matrix, was studied by impregnation in single glass capillaries at the manufacturing temperature of the composites. In order to obtain the net wicking mass for capillary experiments, the meniscus force at the bottom of the glass tube was investigated, with the help of the Wilhelmy technique, conducted by using glass sticks with various diameters. Results show that the contact angles resulting from equilibrium using both capillary impregnation and the Wilhelmy experiment are consistent, which indicates the reliability of the practical evaluation of the effect of the meniscus on the penetration mass. The contact angle of this epoxy resin was approximately 60 resulting from the equilibrium of the capillary impregnation and the Wilhelmy experiments at 80 C. 1. INTRODUCTION In a composite system, the interfacial adhesion between the fibre and the matrix plays an important role in improving the resulting mechanical behaviour, including the strength, modulus and durability of the composite 1. However, good wettability between fibre and matrix is a prerequisite for the occurrence of intimate contact. The behaviour of the spontaneous flow of liquid in a fibrous assembly, driven by capillary forces, is one of the indications of the wettability of fibres 2. Because capillary forces are caused by wetting, wicking is a result of spontaneous wetting in a capillary system. For this reason, various papers have been published on the capillary flow in aligned fibre assemblies in both longitudinal and transverse directions 3-6. One of the experimental methods most frequently used to study this spontaneous flow involves measuring either the increase in weight of tows caused by liquid *Corresponding author. Tel: , Fax: , zgzhang@buaa.edu.cn Rapra Technology Limited, 2006 flow or the wicking height of the penetrated liquid inside the tows. Measurement of the mass of liquid is more convenient, because of the simplicity and accuracy of weight measurements, which can be made by means of a commercial tension measuring device. This technique is proposed in order to avoid the difficulties that arise when the liquid front is not visible or does not reflect the real progression of the fluid 7-9. If we do a summation of all the forces acting on the capillary system that result in an increase in the weight recorded by the electron-balance, then we get Equation 1: W(t) = F film + F wick F buoy, (1) where F film is the net downward force exerted by the fluid film due to the meniscus formed by the free surface of the liquid and the bottom of the solid, F wick is the net downward force caused by the weight of the fluid penetrating the fibrous assemblies, F buoy is the upward buoyant force, and W(t) is the net upward tension in the clip suspended from the microbalance. Of these three forces, only the net wicking weight F wick is the focus of attention. 251

2 Min Li, Zuo Guang Zhang and Zhi Jie Sun The buoyant force F buoy can be neglected if the capillary contact made by the liquid with the liquid container is large enough; if so, the mass increase recorded by the electron-balance can be given by Equation 2: m(t) = m film + m wick, (2) where m film is the mass increase due to the meniscus formed by the liquid film, and m wick the mass increase due to wicking inside the fibrous solid. The schematic of this physical phenomenon is exhibited in Figure 1. Labajos-Broncano et al. have proved that the mass increase caused by the film force has a notable effect on the recorded increase in weight 10. However, few investigators have studied this meniscus at the bottom of the fibres or the holder of the fibre-bank experimentally because of the difficulties caused by the exceptionally small diameter of the fibres, the unknown effective capillary radius, and the immeasurable perimeter of the fibre bundles. In this paper, we present measurements of the equilibrium contact angles between epoxy resin and glass capillaries at high temperature, conducted by using a series of glass tubes with various internal and external diameters. In order to obtain the net wicking mass m wick, the film force m film exerted by the meniscus of the fluid surface was measured experimentally by the Wilhelmy balance method 11, which was conducted by using glass sticks with various diameters. The contact angles resulting from the net wicking mass in the capillary impregnation Figure 1. Schematic diagram of the meniscus at the bottom of a glass tube in the glass capillary experiment are compared with values from the film force in the Wilhelmy experiments at equilibrium. 2. THEORETICAL BACKGROUND In the last few decades, the contact angle has been made the centrepiece of wetting and wicking in numerous published papers However, the contact angle is not the cause of wetting but a consequence of it, as already emphasized by Miller 17. The term contact angle has several meanings and different kinds of contact angle can be used in describing different wetting processes, such as the intrinsic contact angle, the equilibrium (static) contact angle, the dynamic contact angle, the advancing contact angle and the receding contact angle etc. The advancing contact angle is one of the most widely used parameters in discussions of wicking. However, the advancing contact angle is generally larger than the static (equilibrium) one, even for a totally wetting liquid. Hoffman proposed that the value of the dynamic advancing contact angle depends on the liquid penetration rate 18. In this paper, the equilibrium contact angle is employed to evaluate wicking behaviour in single glass capillary experiments. The maximum height reached by the liquid front h E can be given by the balance (Equation 3) between the Laplace pressure and the hydrostatic pressure 19 (3) where θ is the equilibrium contact angle between the liquid and the glass capillary, r the radius of the vertical glass tube, and ρ the density of the liquid. If we replace the equilibrium height h E by the equilibrium mass m E, then a modified form of Equation (3) can be obtained, Liquid front (4) where d is the internal diameter of the glass tube. Meniscus Equation (4) predicts linear behaviour when the equilibrium mass m E is plotted as a function of the internal diameter of the glass capillary. Moreover, the value of cosθ can be calculated from the slope of the straight line, and consequently we can get 252

3 the equilibrium contact angle between the liquid and the glass tube. However, the increase in the weight registered during the experiments is due to the superposition of two physical phenomena: the wicking of liquid in the glass tube, and the formation of a meniscus at the bottom of the tube when it makes contact with the free surface of the liquid 10. For this reason, the experimental increase in weight of the glass capillary must be split into two contributions, m film and m wick, as in Equation (2). The weight increase due to the meniscus at the bottom, m film, can be evaluated by means of the Wilhelmy balance principle. Experiments in this paper were carried out using both the glass tubes with sealed top end, and the glass sticks. The measurable quantity in the experiment is the wetting tension γ L cosθ at the solid/liquid interface, which equals the force F per unit length of the perimeter P of the glass cylinder, recorded by the electron-balance (Equation 5): F = γ L cosθ P (5) The force can be rewritten as F = m film g (6) where m film is the difference in mass before and after the bottom of the glass tube contacts the liquid, and g is the gravitational constant. The perimeter of the contact line between the liquid and the glass tube can be given by P = πd (7) where d is the external diameter of the glass tube or the diameter of the glass stick. If the equilibrium mass gain of the film force m film is plotted as a function of the external diameter d, then we can obtain a straight line (8) with a similar slope to that given by Equation (4). From the slope of the straight line we can get the equilibrium contact angle between the liquid and the glass samples. Therefore, the net wicking mass of liquid sucked into the glass tubes can be obtained by excluding the mass result from the liquid meniscus on the bottom from the raw mass increase of the capillary experiments. The equilibrium contact angle between the fluid and the glass tube can be calculated from the slope of the straight-line plot of m wick versus the internal diameter of the glass capillaries, according to Equation (4). 3. EXPERIMENTAL Capillary impregnation experiments were carried out using a series of single glass tubes with different diameters, hung vertically by a clip from the electronbalance with their lower ends in contact with the testing fluid. The weight increase caused by the wicking was registered by the electron-balance as a function of time. Time zero was the moment of dipping the tube into the liquid container. The force of the liquid meniscus attached to the bottom side wall of the glass tube was measured by using a single glass tube with a sealed top end, or by using a single glass stick according to the Wilhelmy balance method. To avoid confusion, the glass tube with its top end sealed by fusing is termed glass cylinder. All the glass samples, both the tube and the stick used for the investigation, were similar with a length of 100 mm, but different in their diameters. They had been cleaned in an ultrasonic bath three times and dried in air before use. The capillary rise experiments were carried out with the glass tubes. Glass cylinders and glass sticks were employed in the blank Wilhelmy experiments to measure the meniscus force. The electron-balance is a very sensitive weight recording machine (the least count is 0.1 mg, METTLER AB204-S). The measurement was stopped after 6 to 300 seconds, depending on the nature of the glass sample and the test liquid. All measurements were repeated at least three times. In this work, hexane and a pure epoxy resin were used as the test fluids. Table 1 gives the detail, the surface tension and the viscosity of the epoxy resin were measured by the authors, and the others are common values from the manufactures and literatures. The hexane were used in the highest available grade of purity, (supplied by Beijing Chemical Plant), it was 253

4 Min Li, Zuo Guang Zhang and Zhi Jie Sun Table 1. Properties of fluids used in this work Testing liquid Liquid viscosity (mpa s) Liquid density (g cm -3 ) Surface tension (mn m -1 ) Hexane (20 C) Epoxy resin (80 C) 73 a 1.15(20 C) 43.6 a a Parameters measured by the authors employed because of its well known features (surface tension, viscosity, and density), and because it wets the glass completely (θ=0 ). It is normally the test liquid used to measure the packing factor for porous media, such as fibres and powders 1. Both the capillary impregnation and the Wilhelmy experiments of hexane were conducted at ambient temperature. The epoxy resin used in this study was a diglycidylether of bisphenol-a (DGEBA E51, supplied by Wuxi Dic Epoxy Co. China), since it is widely used as the resin matrix in composites. The viscosity of the epoxy resin was measured by using a Brookfield viscometer, and its surface tension was measured using the Dynamic Contact Angle Analyzer apparatus (Dataphysics model DCAT21 system) with the help of the Wilhelmy Principle, using a platinum blade at 80 C. The capillary rise method and the Wilhelmy experiments with the epoxy resin were both performed at 80 C because this gave a relative low viscosity. 4. RESULTS AND DISCUSSION 4.1 Net Wicking Mass in Capillary Impregnation In this section, hexane was used as the testing liquid because of its total wetting characteristics. Firstly, the capillary rise of hexane in a single glass tube was investigated using a tube for which the internal diameter and the external diameter were different from each other (Table 2). The resulting mass gain m at different penetration times is shown in Figure 2. Two different parts can be clearly observed in each experimental curve: the first one, where the slope changed as the time increased; and the second one, at longer times, where m remained constant due to the capillary rise equilibrium in the glass tube. Besides, the equilibrium mass m E of the curves increased with increasing internal diameter of the glass capillaries. As implied by Equation (4), the plot of m E as a function of the internal diameter showed a very straight-line relationship in Figure 4(1). However, the value of cosθ calculated from the slope of this line was 2.49, whereas it ought to be no more than To interpret this behaviour, we must keep in mind that the increase in weight registered during the glass capillary experiments must be split in two contributions, m film and m wick. Consequently, blank experiments were carried out to determine m film, and glass cylinders with their top end sealed by fusing were firstly employed. As revealed by Figure 3, the equilibrium mass increased with increasing external diameter of the glass cylinders. Besides, a plot of the equilibrium mass versus the outer diameter showed a straight-line relationship, as predicted by Equation (8) (see Figure 4(2)). From the slope of the line, the value of cosθ could be deduced to be 1.25, which is too large to be possible. Observation shows that an extra increase in the recorded mass, apart from m film is caused by a small capillary rise in the glass cylinders, which had not been taken into account in our experiments. As a result, the value of m film cannot be measured exclusively by this one end glass cylinder experiment. Table 2. Meniscus force m film for hexane in the glass capillaries experiments calculated according to Equation (8) and the external diameter d EX of the glass tubes d IN (mm) d EX (mm) m film (g)

5 Figure 2. Mass increase m as a function of time t for the capillary rise of hexane in single glass tube with different internal diameters Figure 3. Mass increase of hexane for the blank experiments conducted using a single glass cylinder with different external diameters, and a sealed top end Figure 4. Plots of the equilibrium mass gain as a function of the glass cylinder diameter. (1) Mass of the capillary impregnation versus the internal diameter. (2).Mass of the blank experiment versus the external diameter 255

6 Min Li, Zuo Guang Zhang and Zhi Jie Sun In order to measure the weight increase caused exclusively by the wetting tension m film, another Wilhelmy balance method was employed by using single glass sticks with diameters ranging from 0.72mm to 1.82mm. Figure 5(a) provides details of the results. The mass at equilibrium versus the diameter of the glass sticks is plotted in Figure 5(b). The value of cosθ from the slope of the straight line now equalled 1.00, according to the Wilhelmy balance principle, Equation (8). That is to say, the contact angle θ between hexane and the glass stick was 0, which is coincident with the complete wetting assumption for hexane in most papers. As a result, the meniscus force m film of hexane imposed on the glass capillaries can be related to the outer perimeter πd EX of the tubes, according to Equation (8). Table 2 shows the results for each glass tube in the capillary rise experiments. What is more, we could get the net equilibrium wicking mass m wick for the capillary impregnation by excluding the meniscus force m film (Table 2) from the original results (Figure 2). A plot of m wick as a function of the internal diameter of the glass capillary is shown in Figure 6. The value of cosθ resulting from the slope of this straight line was 0.99, i.e. the equilibrium contact angle between hexane and the glass tube was about 8, which agrees approximately with the value calculated from the Wilhelmy experiment with glass sticks. Figure 5. Mass increase due to the meniscus m film for the Wilhelmy experiments with single glass sticks and hexane as the testing liquid: (a) weight change at different times; (b) the weight at equilibrium as a function of the external diameter of the glass sticks 256

7 To sum up, the meniscus force m film has a very important effect on the mass recorded for the single glass capillary impregnation. The equilibrium contact angle between hexane and glass samples is approximately zero, which provides a practical evaluation of the meniscus force in order to obtain the net wicking mass in penetration experiments. 4.2 Contact Angle of Epoxy Resin at High Temperatures Capillary impregnation of the epoxy resin in single glass tubes was studied at 80 C. Equation (4) indicates a straight-line relationship between the equilibrium mass gain m E and the internal diameter d IN of the glass tubes for this case. However, the plot of m E versus d IN did not reveal a distinct straight line in Figure 7(1). From the curve, we found that the experimental weight of a glass capillary with thicker walls appears to be much larger than the weight of the tubes with thinner walls; the diameter of the glass tubes is exhibited in Table 3. Therefore, the contribution of the meniscus force to the penetration mass of epoxy resin in a single glass capillary is too large to be neglected, just as for hexane. Figure 6. Plot of the net wicking mass m wick of hexane in the single glass capillary experiments as a function of the internal diameter of the glass tubes Figure 7. Plot of the mass at equilibrium as a function of the diameter, with epoxy resin used as the testing liquid at 80 C. (1) Equilibrium mass gain for the capillary experiments versus the internal diameter of the glass tube (2) m film due to the meniscus force versus the external diameter of the glass sticks in the Wilhelmy experiments 257

8 Min Li, Zuo Guang Zhang and Zhi Jie Sun The meniscus force of the epoxy resin was tested according to the Wilhelmy method using glass sticks at 80 C with the aim of obtaining the net wicking mass in capillary experiments. The data for m film due to the meniscus force were plotted as a function of the diameter of the glass sticks in Figure 7(2), which reveals an excellent straight line. Consequently, we could get the contact angle between epoxy resin and the glass sticks from the slope of the plot; the value was 61. Moreover, m film for the capillary rise experiments with epoxy resin could be calculated according to Equation (8) and the external diameter of glass tubes. Table 3 gives the details. Finally, we obtained the net wicking mass m wick, having excluded the meniscus force m film in single glass capillary experiments at equilibrium. A plot of the net wicking mass m wick as a function of the internal diameter is displayed in Figure 8. The contact angle between epoxy resin and the glass tube was 59, calculated from the slope of the straight line in accordance with Equation (4). This is approximately coincident with the value obtain from the Wilhelmy experiment, i.e., the equilibrium contact angle of epoxy resin was about 60 for both experiments at 80 C. 5. CONCLUSIONS Epoxy resin is one of the most widely used resins in the field of fibrous composites. The contact angle of epoxy resin was investigated at a typical fabrication temperature by capillary rise experiments, carried out using the increase in weight technique. We have also shown that meniscus formation has a great effect on the recorded mass when the bottom of the glass capillary is put in contact with the free surface of the penetration liquid. The meniscus force was measured by a single glass stick with different diameters according to the Wilhelmy balance principle, in order to determine the wicking mass. Hexane was employed as a complete wetting fluid in these experiments at ambient temperature. Its equilibrium contact angle is zero according to the Wilhelmy experiment, and the value from the net Table 3. Meniscus force m film for epoxy resin in capillary experiments according to Equation (8) and the external diameter d EX of the glass tubes d IN (mm) d EX (mm) m film (g) Figure 8. Plot of the net equilibrium wicking mass of epoxy resin for capillary impregnation versus the internal diameter of the glass tube at 80 C 258

9 wicking mass of the capillary impregnation is 8. These results are consistent with each other within the attainable precision of the experiments. Furthermore, this represents a practical evaluation of the effect of the meniscus force on single capillary penetration. The equilibrium contact angle of epoxy resin for the Wilhelmy method was 59, and 61 for the capillary impregnation of glass tubes, which are approximately coincident with each other. Therefore, the contact angle of epoxy resin is approximately 60 at the test temperature of 80 C. ACKNOWLEDGMENT This work has been funded by the 863 Program of National Committee of China under Project No. 2001AA REFERENCES 1. Park, S.J., Kim, M.H., Lee J.R. and Choi, S., Colloid and Interface Science, 228, 287, (2000). 2. De Boer, J.J., Textile Research, 15, 624, (1980). 3. Batch, G.L., Chen Y.T. and Macosko, C.W., Reinforced Plastics and Composites, 15, 1027, (1996). 4. Amico, S.C. and Lekakou, C., Polymer Composites, 23(2), 249, (2002). 5. Tavisto, M., Kuisma, R., Pasila, A. and Hautala M., Industrial Crops and Products, 18, 25, (2003). 6. Krishna, M.P. and Suresh G.A., Colloid and Interface Science, 183, 100, (1996). 7. Pezron, I., Bourgain, G. and Quéré, D., Colloid and Interface Science, 173, 319, (1995). 8. Vernhet, A., Bellon-Fontaine, M.N., Brillouet, J.M., Roesink, E. and Moutounet, M., Membranes Science, 128, 163, (1997). 9. Varadaraj, R., Bock, J., Brons, N. and Zushma, S., Colloid and Interface Science, 167, 207, (1994). 10. Labajos-Broncano, L., González-Martín, M.L., Bruque J.M. and González-García C.M., Colloid and Interface Science, 234, 79 (2001). 11. Grundke, K., Uhlmann, P., Gietzelt, T., Redich, B. and Jacobasch, H.-J., Colloids and Surfaces A: Physicochemical and Engineering Aspects, 116, 93, (1996). 12. Barsberg, S. and Thygesen, L.G., Colloid and Interface Science, 234, 59, (2001). 13. Rebouillat, S., Letellier, B. and Stiffenino, B., Adhesion & Adhesives, 19, 303, (1999). 14. Silva, J.L.G. and Al-Qureshi, H.A., Materials Processing Technology, 92, 124, (1999). 15. Siebold, A., Nardin, M. and Schulta, J., Colloid and Surface A, 161, 81, (2000). 16. Lee, Y.N. and Chiao, S. M., Colloid and Interface Science, 181, 378, (1996). 17. Miller, B. and Young, R.A., Textile Research, 45, 359, (1975). 18. Hoffman, R.L., Colloid and Interface Science, 50, 228, (1975). 19. Raphaël, E., Physics, 50, 485, (1989). 259