Hydrophilicity can be thought of as an opposite to hydrophobicity; hydrophilicity is the attraction between water and a surface. It has wide-ranging implications that affect bandages and medical dressings, clothing, and any other absorbent textiles. Whereas few textiles are inherently hydrophobic, many textiles are inherently Hydrophilic?. This property can be enhanced by physical or chemical treatments, or can be suppressed to make a material more hydrophobic. Hydrophilicity is a driving force behind wicking, which is essential for comfort in clothing or dressings. It also determines or influences Moisture regain?, drying times and absorbency in textiles.
Hydrophilicity is of relevance to all textile markets sectors. It impacts on the thermal and touch comfort of clothing, the absorbency of non-woven cleaning cloths, the absorbency of bandages and dressings, the performance of floor-coverings and upholstery, or the function of geotextiles. Any textile that comes into contact with water could benefit from a tuned hydrophilicity.
Regulations and Legislation
There are no standards or legislation devoted to hydrophilicity, though there are tests for assessing wicking performance, which is determined, in part, by the hydrophilicity of a material. A relevant wicking test which has its own standard is BS3424-18:1986 . Another method to assess hydrophilicity is by measuring the Contact Angle? of a surface. This is discussed in Hydrophobicity
Terms and Definitions
Neither ASTM, the Textile Institute, nor the British Standards Institute, define hydrophilicity. However, ‘hydros’ refers to water, and ‘philia’ means to love. Therefore, hydrophilicity literally means water-loving, the opposite of water-hating (hydrophobicity). IUPAC (International Union of Pure and Applied Chemistry) define it as “the capacity of a molecular entity or of a substituent to interact with polar solvents, in particular with water, or with other polar groups.”
Wicking is defined as “the passage of liquids along or through a textile material or the textile element of a coated fabric or along interstices formed by the textile element and the coating polymer of the coated fabric.”
There are two major influencing factors on hydrophilicity in textiles: one is the chemical makeup of the fibre, or more specifically, the fibre surface chemistry; and the second factor is the texture of the fibre and fabric.
One of water’s fundamental properties is its ability to take part in hydrogen bonding (described in Oleophobicity). Hydrophilic? materials can also take part in hydrogen bonding, which means that all or part of their chemical structure must be polar. For example, cotton and wool are both cellulosic fibres (the chemical structure of cellulose is shown in Figure 1) and so contain many oxygen molecules and hydroxyl Moieties that can interact with water. The key point is that water will interact favourably with polar groups, for example –OH, R-Halogen, =O, and -NH2. Water interacts less favourably with non-polar materials, for example oils like petrol, or oil-based polymers like polypropylene.
Figure 1 – Cellulose
Polypropylene is naturally hydrophobic because it contains no polar Moieties (see Figure 2). As such, for it to absorb moisture it must be chemically or physically altered.
Figure 2 - Polypropylene
If the Contact Angle? (see Hydrophobicity for more information) between water and a surface is less than 90 ° then the surface is Hydrophilic?. Because the water is attracted to the surface it spreads out. In a fibre, this may progress to wicking, which is driven by capillarity (also known as capillary action or capillary pressure). Capillary Forces are responsible to drive the liquid in capillary spaces. (Click here for more information)">capillary wicking occurs when Adhesion Forces between the liquid molecules and the surface (eg. a fibre), exceed the Cohesion Forces between liquid molecules. Because of the greater affinity between the liquid and the surface, the liquid attempts to spread across the surface. Capillary Forces are responsible to drive the liquid in capillary spaces. (Click here for more information)">capillary wicking is spontaneous and relies on the presence of a porous material (eg. a textile) and a wettable (ie. Hydrophilic?) surface. Surface Tension? (explained under Hydrophobicity) means that the surface of the liquid cannot escape the bulk liquid and so the whole liquid spreads over the surface. This leads to the concave Meniscus that is seen in a glass of water: the water adheres to the glass and spreads up the glass. If the glass were hydrophobic then the Meniscus would be ‘upside-down’ (convex) as the liquid attempts to avoid all contact with the repellent surface. This is shown in Figure 3:
There are two types of wicking: planar wicking and transverse wicking. Planar wicking occurs vertically and horizontally in the plane of a fabric, for example across the width of a fabric. Transverse wicking happens between planes, for example across the thickness of a fabric, from its inside to its outside.
There are numerous tests for determining wicking performance. Two basic tests to measure planar wicking are described in Figure 4 and Figure 5:
Figure 4 - Virtical Strip Test
Figure 6 - Dowbward Strip Test
In the vertical strip test (Figure 4) a sample is placed in a water bath and the distance that the water travels in a certain length of time is measured. However, this test cannot be used to reliably determine the performance of a fabric because the speed of wicking varies according to the experimental coordinate (how much of the experiment has elapsed). The downward strip test (sometimes called the siphon test, Figure 5) is a significant improvement because it removes the effect of gravity. The test must be carried out on at least two samples as the permeability, k, of the fabric is unknown.
Tests for transverse wicking include the gravimetric absorption test, Figure 6, which uses a weight to increase transfer of water. The amount of water that escapes the wetted sample is measured. This test formed part of the standard ASTM D-5802, though this has been withdrawn.
Figure 6 - Gravimetric Absorbtion Test
SDL have developed a machine that will test wicking and all other moisture management properties of a fabric. Though expensive it could be extremely useful in research of this area.
There are numerous equations related to hydrophilicity, wicking and fluid flow. Some of the more commonly used ones are the Lucas-Washburn equation, the Poiseuille-Hagen equation and the specific permeability equation:
Equation 1 - Lucas-Washburn equation, where h = height, C = property of liquid and fabric, t = time
The Lucas-Washburn equation predicts the distance travelled by wicking by a point in time.
dh = ∏r4 ∆p
dt 8n n
Equation 2 - Poiseuille Hagen equation, where h = height, t = time, r = fibre radius, = dynamic fluid velocity, is pressure difference
The Poiseuille Hagen equation is used to predict wicking rate if the fabric is modelled as a series of cylindrical fibres.
k = nhv
Equation 3 - specific permeability equation, where k = specific permeability, n= dynamic fluid velocity, h = height, = viscosity of fluid, p = pressure
The specific permeability, k, of a material represents the void capacity in m2 through which a fluid can flow. It is an important fabric structural parameter that is important to measure if trying to optimise wicking performance and hydrophilicity.
To increase wicking rate at the fibre level there are three basic physical alterations that can be made:
• Decrease fibre diameter
• Increase fibre cross-sectional area
• Increase fibre orientation in direction of transport
The Coolmax fibre is a good example of a fibre with high cross-sectional area, enhancing its ability to wick.
Methods to Increase Hydrophilicity
To increase hydrophilicity then a fibre can be altered physically or chemically. Polypropylene, for example, is often chemically treated. Various chemical methods are summarised in Table 1:
Active gas treatments
Gas plasma discharge (an oxidation technique)
Plasma deposition (of Hydrophilic? gases or vapours) (briefly discussed in Hydrophobicity)
Reaction with ozone (gas phase oxidation)
Reaction with oxidising agents (eg. sulphuric acid and potassium permanganate) in liquid phase
Reactions with surface hydroxyl groups (eg. using thionyl chloride to add chloride functionality)
Use of surfactants
Physical deposition of PEO-based surfactants (PEO is polyethylene oxide)
Chemical bonding (eg. of PEO-X, where X is a functional group)
Desorption of block surfactant from a bulk phase (eg. PEO-silicone)
Radiation-initiated polymerisation (gamma-ray, electron beam, UV)
Table 1(adapted from A. S. Hoffman),
http://onlinelibrary.wiley.com/doi/10.1002/masy.19961010150/abstract ) **(I’m not sure if this needs to say ‘adapted from’ or whether it just needs to be referenced: I’ve used the titles from their table (page 448) but have rewritten all the content)**
Most of the techniques in Table 1increase hydrophilicity by oxidising the fibre. Oxidation is a loss of electrons[i] and often coincides with an increased number of oxygen molecules in the polymer. Oxygen molecules increase polarity in the polymer, making it more likely to hydrogen bond to water.
Challenges and Innovation
Cellulose-based fibres tend to be fundamentally Hydrophilic? and increasing this natural tendency by physical methods, such as brushing and raising, is well understood (LINK REQUIRED). Synthetic fibres, however, tend to require chemical treatments to increase their hydrophilicity. Chemical treatments do not always last as long as physical treatments, as they sometimes only coat the fibre surfaces and are thus worn away when the fibres degrade. In addition, many of the chemical treatments use toxic chemicals that are cheap to use but dangerous and environmentally damaging. Use of potassium permanganate is declining but its use should decrease further wherever possible: it can cause spontaneous fires and is extremely harmful to aquatic life, which is of particular concern when waste-waters are not treated properly. Permanganate is often used alongside sulphuric acid, the dangers of which are well known (LINK REQUIRED). Catalytic oxidation of textiles fibres is not trivial, so some of the alternative methods listed above, such as the use of active gas treatments should be explored.
Novel fibre shapes continue to be developed that display incredible wicking. 4DG fibres, remain amongst the more complicated wicking fibres, but increasing further on their surface area is possible, and could further increase wicking without resorting to further chemical treatment.