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Hydrophobicity

Introduction

Hydrophobicity is a property inherent to some materials, and that can be imparted to others. It relates to how water interacts with a surface of a material, and whether this water is repelled or attracted. Oil is hydrophobic, resulting in the rainbow-like sheen that can be seen when they come into contact with one another. Tissue paper, however, is Hydrophilic? (the opposite to hydrophobic) and therefore absorbs water readily.

Hydrophobicity is driven by the fundamental physical properties of Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy and Surface Tension?. Water behaves differently to almost all other materials, resulting in the great energy differences between a hydrophobic and a Hydrophilic? surface.

Hydrophobicity can be determined using various laboratory techniques and there is an ever-increasing number of methods used to impart hydrophobicity to a surface. In particular, great interest focuses on the creation of superhydrophobic surfaces, and these have uses in textiles as diverse as waterproof materials and self-cleaning surfaces.

Relevant Sectors

Hydrophobicity is of relevance to most – if not all - market sectors, as modifying the way textiles interact with water is fundamental to the way many technologies work. Hydrophobicity is particularly important in clothing, where clothes that dry fast or resist wetting are desired. In addition, carpets or upholstery (Interiors and Automotive market sectors) that resist wetting are easy to care for and tend to resist staining.

Regulations and Legislation

There are few regulations that govern hydrophobicity, though there are numerous standard testing methods. The standards BS ISO 23232:2009 and ISO 23232:2009 provide a guide for aqueous stain resistance. The standards BS EN 29865:1993 and ISO 9865:1991 describe the Bundesmann rain test, which measures the repellence of a textile by dropping water on it. A rating between 1 and 5 is given to the textile.

Terms and Definitions

Neither ASTM nor the Textile Institute define hydrophobicity. Furthermore, few standards define it. However, ‘hydro’ refers to water, and ‘phobic’ means to hate. Therefore, hydrophobicity can be thought of as water-hating. It is the opposite of hydrophilicity (water-loving). Several terms related to hydrophobicity are defined by other sources: aqueous repellency is defined in BS ISO 23232:2009 and ISO 23232:2009, related to textiles, as a “characteristic of a fibre, yarn or fabric whereby it resists absorption by aqueous liquids”. In addition, water repellency is defined as “the relative degree of resistance of a fabric to surface wetting, water penetration, water absorption or any combination of these properties”. A water repellent finish is “a state characterised by the non-spreading of a globule of water on a textile material.” 

 

Basic Principles

When a liquid is in a container the bulk liquid molecules, those surrounded solely by other liquid molecules, are subject to different forces to those at the surface, which are partly in contact with air or another medium. The intermolecular forces acting in the bulk are spherically equivalent – they are the same in all directions. However, at the surface these forces only act in the direction of the bulk liquid, which has the effect of pulling the surface molecules back into the bulk. As a result, the liquid surface contracts to form the smallest possible surface area, for a given volume. This is what gives rise to Surface Tension? and explains the behaviour of hydrophobic materials. When water comes into contact with a hydrophobic surface, such as clean glass, the water forms a spherical shape so as to reduce its surface area to a minimum. Related to this property is capillarity, which is discussed under hydrophilicity.

Wetting of a textile surface involves three stages: immersion, adhesion, and spreading. During immersion, the solid/vapour interface is replaced by a liquid/solid interface. Adhesion, a property fundamentally related to hydrophobicity, is determined by the change in surface free energy in the system, and cohesion is determined by the energy required to pull apart a liquid column to create two new surfaces (ie. how much does the liquid want to remain as a whole). Surface free energies,  that determine adhesion, cannot be measured; instead, contact angles are measured to determine hydrophobicity. Contact angles are the angle that is formed when a liquid comes into contact with a solid. Contact angles are described by Equation 1 and Figure 1:

Hydrophobicity Formula 1

Equation 1

Hydrophobicity 1

Figure 1

In Equation 1, YSA  is the solid-air Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy,  YSL is the solid-liquid Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy, and  YSA is the Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy between solid and air. The Contact Angle?, ØC determines whether the liquid will wet the surface or not. Assuming that the liquid is water, a Contact Angle? greater than 90 ° indicates a hydrophobic surface. If the Contact Angle? is less than 90 ° then the surface is Hydrophilic?.

Imparting Hydrophobicity

Hydrophobicity is inherent to some materials, but few untreated textiles are hydrophobic. Polypropylene is certainly more hydrophobic than cotton, but its Contact Angle? is still relatively low. Hydrophobicity can quite easily be imparted on a surface, however. This can be achieved either through a physical or chemical modification of the material surface. Repellent finishes (chemical modification) work by imparting low Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy to a material, and making it lower than the Surface Tension? of the liquid. Water has a Surface Tension? of approximately 73 mN m-1 (millinewtons per metre), so hydrophobic finishes must exhibit a lower Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy than this. Examples of hydrophobic finishes are polysiloxaneshttp://www.eng.buffalo.edu/Courses/ce435/Polysiloxanes/,  fluorocarbonshttp://en.wikipedia.org/wiki/Fluorocarbon,  and waxes. To impart oil-repellency is challenging, as the surface tensions of oils are lower than that of water.

Waxes were one of the first materials used to impart hydrophobicity on textile surfaces. They are still used today in clothing such as Barbour jackets. Silicones (-O-Si-O polymers) are in common use and are applied by Padding? then curing at high temperature. Fluorochemical finishes repel water readily as they have very low Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy. They are also durable, but must be applied in an energy-intensive, high temperature, process. They are also environmentally detrimental.

Challenges and Innovation

For approximately 15 years there has been widespread research into superhydrophobicity (surfaces with a Contact Angle? greater than 150 °) and ultraphobicity (surfaces with a Contact Angle? greater than 160 °). Much of this research has focused on biomimicry of the lotus leaf, butterfly wings, and the feet of water striders, which each exhibit superhydrophobicity. Superhydrophobicity can be imparted by roughening a low Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy material, or by lowering the Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy of a rough material. These methods leave great room for innovation, and new methods continue to develop. For example, in 2010 it was found that galvanic deposition  could be used to create superhydrophobic surfaces with very little economic outlay. Hydrophobic surfaces are of great interest to the textile industry because of the prospect of self-cleaning materials, fast-drying textiles, and stain resistance.

A major challenge for hydrophobicity is the durability of the chemical finishes: even the highest quality fluorochemicals last only for short periods if subjected to continuous abrasion. Once abraded they are difficult to reapply and are rarely as effective as when new. Plasma, (See video link below) a relatively new method of imparting hydrophobicity, has been used to prepare very impressive hydrophobic surfaces, though there is concern over how long these coatings will last. A further challenge for the field is reducing the environmental impact of fluorochemicals. As they are the highest performance of the conventional chemical finishing methods, their use is widespread, despite their strong greenhouse effect and bioaccumulative nature that may be of danger to both animals and humans.