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Hydrophilicity

Introduction

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.

Relevant Sectors

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.

 

(Hydrophilicity cont:)

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.”

 

Basic Principles

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 Moietieshttp://en.wikipedia.org/wiki/Functional_group 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.

Cellulose 1

Figure 1 – Cellulose

Polypropylene is naturally hydrophobic because it contains no polar Moietieshttp://en.wikipedia.org/wiki/Functional_group (see Figure 2). As such, for it to absorb moisture it must be chemically or physically altered. 

Cellulose 2

Figure 2 - Polypropylene

 

Wicking

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 Forceshttp://en.wikipedia.org/wiki/Capillary_action are responsible to drive the liquid in capillary spaces. (Click here for more information)">capillary wickinghttp://www.thesmarttime.com/articles/wetting-wicking-3.htm occurs when Adhesion Forceshttp://en.wikipedia.org/wiki/Adhesion between the liquid molecules and the surface (eg. a fibre), exceed the Cohesion Forceshttp://en.wikipedia.org/wiki/Adhesion between liquid molecules. Because of the greater affinity between the liquid and the surface, the liquid attempts to spread across the surface. Capillary Forceshttp://en.wikipedia.org/wiki/Capillary_action are responsible to drive the liquid in capillary spaces. (Click here for more information)">capillary wickinghttp://www.thesmarttime.com/articles/wetting-wicking-3.htm 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 Meniscushttp://en.wikipedia.org/wiki/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 Meniscushttp://en.wikipedia.org/wiki/Meniscus would be ‘upside-down’ (convex) as the liquid attempts to avoid all contact with the repellent surface. This is shown in Figure 3:

Wicking Fig 3

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:
 

Virtical Strip Test  Fig 4

Figure 4 - Virtical Strip Test

Downward Strip Test Fig 5

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.

Gravimentric test Fig 6

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:

h=Ct½

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
        p

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 Coolmaxhttp://coolmax.invista.com/ fibre is a good example of a fibre with high cross-sectional area, enhancing its ability to wick.

(Back to Contents )

 

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

Corona discharge (in air)[i]

Gas plasma discharge (an oxidation technique)

Plasma deposition (of Hydrophilic? gases or vapours) (briefly discussed in Hydrophobicity)

Chemical reactions

Reaction with ozone (gas phase oxidation)

Reaction with oxidising agents (eg. sulphuric acid and potassium permanganate) in liquid phase

Reactions with functional groups (e.g. with anhydride compounds)[ii]

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)

Inert radio frequency glow discharge (RFGD)[iii] surface crosslinking of surfactants

Desorption of block surfactant from a bulk phase (eg. PEO-silicone)

Graft polymerisation[iv]

Radiation-initiated polymerisation (gamma-ray, electron beam, UV)

Chemically-initiated polymerisation

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.

Physical methods of increasing hydrophilicity tend to increase the surface area of the fabric. Thus, methods include brushing and raising, discussed under Dyeing and Finishing are frequently.

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 oxidationhttp://www.ncbi.nlm.nih.gov/pubmed/21553036 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.