Is it important to know the surface free energy of textiles? Methods to characterize textile/fibre surface for different treatments like printing, dyeing, coating etc. R. Minch Krüss GmbH, Borsteler Chaussee 85, 22453 Hamburg, Germany. Email: [email protected] Wettability and adhesion as well as water- or dirt-repellent properties are determined by surface energy. This plays a key role in problems concerning hydrophilicity/hydrophobicity and optimization of printing, coating, dyeing and cleaning of textiles. This presentation describes existing practical ways of surface free energy determination on different textile surfaces like fibres, fibre bundles, woven/nonwoven textiles and fabrics. There are two different measurement techniques which can be applied for this purpose: optical and non optical. The optical method is based on standard sessile drop technique and can be used mostly for fabrics. The study of wetting behaviour of textiles using this method can hardly be influenced due to unevenness of the sample and rapid ad- or absorption of test liquids. This can be remedied by the captive bubble method. Non optical methods are based on using balance based tensiometers. Here two different methods can be distinguished as well: dynamic Wilhelmy method which is usually applied for single fibres and Washburn method which can be applied for characterization of fibre bundles and woven textiles. Advantages and limitations of these methods will be discussed. 1. Why we need to know surface free energy of fibres and/or textiles? Fibres affect all of us in one way or another during our daily lives. They may be used in the form of a composite material, a woven or non-woven technical textile or even in reinforced concrete structures. The surface properties of these fibres play a key role in delivering the performance required by a particular process or final product. To impart the fibre with the appropriate surface properties, surface treatments or coatings are applied and most often serve to protect, functionalise, lubricate, colour code and sometimes decorate the surface. Generally, coating is achieved by drawing a fibre at a given speed through a bath containing the coating liquid. As the fibre traverses the liquid air interface, a thin layer of coating is deposited onto it. The quality of this film depends on fibres wettability. The wetting of fibres with surfactants at different concentrations both below and above the CMC has been the subject of a number of studies. It has been generally observed that the presence of surfactant in a solution at concentrations below the CMC leads to a thickening of the film-coating compared to that of the pure solution. But the influence of surfactants is not restricted to a simple increase of the film thickness with increased surfactant concentration. In fact, for surfactant concentrations close to and above the CMC, the film thickness can decrease with increasing surfactant concentration. This has been reported for both surfactant solutions of SDS [1] and Triton X100 [1]. In fibre coating and textile treatment applications, the aim is typically to improve the wetting and adhesion between the fibre/textile and the coating liquid or ink. This can be achieved by modifying the surface characteristics of the fibre surface or by adjusting the formulation of the coating liquid or inks. In either case, this will lead to a change in the surface energy (and probably the surface polarity) of the respective phase. The surface tension of a dye must be matched exactly to the corresponding parameter of the solid; if the energy of the solid is too low then some areas will not be wetted, if it is too high then coating defects such as "orange peel", "picture framing", etc. can occur. In most cases the phenomenon of adhesion can be explained directly by the surface free energies of the two partners involved. In this case knowledge of polar part is particularly necessary. As a simple criterion for optimal adhesion the complete compatibility from an energetic point of view as well as the presence of as large a polar part as possible on both sides is necessary. This means that the total surface free energies – both the dispersive as well as the polar parts of both phases in particular – should be identical and as high as possible. From surface energy values, two important, guiding, interfacial parameters are gained: physico-chemical adhesion and coating/substrate interfacial tension. These are described as follows. Assuming a simple two component (geometric mean) surface energy approach, such as Fowkes or Owens-Wendt, the predicted adhesion energy (ΨSL) between a liquid (coating) and substrate (fibre) is given by Fowkes/Dupré expression: D D 1/2 P P 1/2 ΨSL = 2(γ S γ L ) + 2(γ S γ L ) (1) D P D Where γS and γS are disperse and polar components, respectively, of the substrate and γL P and γL are disperse and polar components of the liquid. The total surface energy of either material equals the sum of disperse and polar surface energy components. The surface polarity is given by the percentage of the overall surface energy that is due to the polar surface energy component. The interfacial tension (γSL) between the substrate and the coating is given by Good’s expression: D D 1/ 2 P P 1/ 2 γ SL = γ S + γ L − 2(γ S γ L ) − 2(γ S γ L ) (2) Where γS is equal to the total surface energy of the substrate and γL the total surface tension of the coating. A more detailed treatment of the theoretical backgrounds and developments of these equations has already been reported [2]. The two properties, adhesion energy and interfacial tension are closely related. By combining the two previous equations we can show that adhesion energy (ΨSL) simply equals the sum of the energies of the two surfaces being brought together, less the interfacial tension which remains in the bond which is formed between them. ΨSL = γ S + γ L − γ SL (3) Despite being inherently linked mathematically, adhesion energy and interfacial tension are different parameters. Depending upon the application in question, interfacial tension values can provide different and sometimes more pertinent information than adhesion energy. The difference between the two is this: Adhesion energy tells how energetically favourable the initial formation of an interface (fibre/coating; textile/inc) is. In forming a bond, you sacrifice substrate (fibre, textile) surface and coating (liquid, inc) surface and you create an interface. Interfacial tension on the other hand, has less to do with the original surface energies of the substrate (solid) and the coating (liquid) which form the interface (though it is influenced by these surface energies). It is fundamentally a property of the bond after it is formed. When two materials come together, they sacrifice their surface energies to form an interface. Adhesion energy describes the energetic favourability of interface formation whereas interfacial tension describes the incompatibility which is left over once the interface is formed (the tendency for the bond to break if stress is applied). Therefore, the knowledge of surface free energy of textiles split into polar and disperse parts can be used for optimizing coatings and inks properties. Their chemical composition can be changed appropriate to assure good wettability and adhesion. To determine the surface free energy of fibres and/or textiles split into disperse and polar parts the contact data (cosθ) of solid to be studied (fibre or textile) with liquid of known D P properties (γL ; γL ) are required. According to Young’s equation: γ S = γ SL + γ L ⋅ cosΘ (4) From this equation the interfacial tension solid – liquid (γSL) is equal: γ SL = γ S − γ L ⋅ cosΘ (5) Combining equations (2) and (5) and using OWRK (Owens, Wendt, Rabel and Kaelble) D P model the components of surface free energy of solid can be determined (γS ; γS ). In this paper different methods which can be applied for determining the contact angle values with their limitations will be demonstrated and discussed. 2. Non-optical methods for contact angle determination. 2.1. Single Fiber Contact Angle Measurement The “traditional” method of measuring contact angle on such fibers is the Wilhelmy Advancing Contact Angle - Wilhelmy Method Fiber Force, mN Air Liquid Liquid moving θa up Receding Contact Angle - Wilhelmy Method Fiber Liquid Force, mN moving Air down Liquid θr Figure 1. Advancing and receding contact angle determination for single fibres using Wilhelmy method. technique, using a single fiber. This technique requires a balance based tensiometer. A single fiber is dipped into a non-penetrating liquid, last wets the fiber and the force due to wetting is measured. An advancing (wetting) contact angle for the liquid on the fiber is determined from force data obtained during submersion of the fiber into the liquid. A receding (dewetting) contact angle is likewise determined from force data pertaining to the removal of the fiber from the liquid (Fig. 1). Contact angles are calculated from force data using the Wilhelmy equation. F − F cosΘ = b lγ L (6) wherein θ - the contact angle, l - the fiber’s wetted length (circumference), γL - the surface tension of liquid, F - the total force felt by the fiber at any submersion position, and Fb - the buoyant component of the force on the fiber at any submersion position. Fb is due to the fiber displacing liquid as it is submerged and removed. It is, in general, a distraction to Wilhelmy contact angle experimentation, since the contact angle depends on the Wilhelmy force (Fw) only, which is the wetting force, or the total force felt by the solid (F) less the buoyant force (Fb). Hence, Fw = F - Fb as used in the Wilhelmy equation above.