Breathable polyurethane membranes for textile and related industries

George Robert Lomax

Received 6th March 2007, Accepted 8th May 2007

First published as an Advance Article on the web 22nd May 2007

DOI: 10.1039/b703447b

This Application article briefly reviews the science behind so-called ‘‘breathable’’ coatings and

films, designed originally for foul-weather garments. These materials form continuous polymer

layers over a textile substrate, which are impermeable to liquids such as rainfall and yet transmit

water vapour emitted from the body. Some bear many similarities to dense membranes used for

ultra-filtration and reverse osmosis. Hydrophilic polyurethanes containing poly(ethylene oxide),

PEO-PUs, form a distinct class that straddle the ill-defined boundaries between conventional

macromolecular chemistry, nanotechnology, biomimetics and smart materials. The unique

properties and bio-compatibility of PEO provide further opportunities for these polymers in

diverse areas such as packaging, medical devices, controlled-release systems, hydrogels,

electronics, and biosensors.

1. Introduction

Water vapour-permeable polymer membranes have been used

in sports apparel, fashion rainwear and protective workwear

for more than thirty years.1,2 Their main application is in

waterproof, breathable fabrics (WBFs) for anoraks, over-

trousers, survival suits etc., which help to alleviate problems

of heat build-up and condensation as the wearer becomes

more active.3 Increasingly therefore, WBFs play an important

role in outdoor clothing systems based on interactive layering

and total moisture-management principles.4,5 The aim here is

to provide the best possible compromise between comfort

and protection, under the widest range of activities and

climatic conditions. Non-apparel uses include shoe panels

and inserts, tents, bivouac bags, horse blankets, boat

tarpaulins and medical items such as surgeons’ gowns, drapes,

bodily fluid-proof and anti-allergy mattress covers, and wound

dressings.6

Membranes used in WBFs are typically 10–30 mm thick, and

can discriminate between water droplets (rain or other forms

of atmospheric precipitation, diameters .100 mm) and

moisture vapour or perspiration (individual water molecules,

y0.3 nm diameter), essentially on the basis of size.7 They fall

into two main categories: microporous membranes that have a

clearly visible system of holes and interconnected passage-ways

and hydrophilic membranes that do not, Fig. 1. In some types

of WBF, these two structures are combined.8

Two- and three-layer WBFs are manufactured as con-

tinuous rolls, typically 1.5–2.0 m wide and 100–5000 m long,

by well-established techniques of direct coating (with solu-

tions, dispersions and molten compounds), transfer coating

using siliconised release papers or belts, and film lamination.6,9

Tightly woven polyester and polyamide fabrics are the normal

substrates, increasingly made from textured yarns that

simulate more luxurious natural fibres. It is fair to say that

some of the earliest 2-layer garments lacked finesse. Often, the

WBF was relatively stiff and crackly, they had stitched seams

that were visible and prone to leakage, and they were unlinedBaxenden Chemicals Ltd, Applied Chemicals Division, Union Lane,Droitwich, UK WR9 9BB. E-mail: robert.lomax@baxchem.co.uk;Fax: +44-1905-794002; Tel: +44-1905-794795

George Robert Lomax

Robert Lomax holds B.Sc. andPh.D. degrees in chemistryfrom the Victoria University ofManchester. In 1979 he joinedShirley Institute, Didsbury andworked on various topics, suchas novel hydrophilic polymers,and studies on fabric wicking,absorbency and breathability.From 1990 onwards, he hasbeen employed by BaxendenChemicals in PU applications,research and technical salesroles, and is currently BusinessDevelopment Manager.

Fig. 1 SEMs of breathable membranes used in WBFs: (a) section

through an asymmetrically porous PU coating on nylon taffeta fabric

(61 800 magnification); (b) section through a non-porous, hydrophilic

PU coating on polyester/cotton fabric (6550), indicating the

successive coating layers.

APPLICATION www.rsc.org/materials | Journal of Materials Chemistry

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so that the polymer membrane could come into direct contact

with the skin. Since the 1980s, technologies for incorporating

membranes into fabrics and garments have evolved dramati-

cally,1 see Fig. 2. Nowadays, their aesthetics (tactility, handle,

drape and visual appearance) are minimally impaired, e.g. in

configurations Fig. 2(d), (e), or can at least be offset against

cost and customer choice. Inner linings help wicking of

perspiration, and take away the ‘‘wet cling’’ sensation of

membranes against skin. Optionally, the exposed membrane

surface can be bonded to e.g. lightweight tricot or insulating

materials to give 3-ply laminates, Fig. 2(g), which were first

developed for top-end sports and fashion wear, and are now

routinely used in high-visibility workwear.

2. Water vapour diffusion through membranes

It must be noted at this point that breathability testing of

WBFs is fraught with controversy and confusion.8,10 This

applies particularly to evaluation of commercial fabrics in the

laboratory, and extends to wearer trials of garments in climatic

chambers or in the field. The basic problem stems from the

difference in water vapour transport mechanisms between

microporous and hydrophilic layers, exacerbated by swelling

effects in the latter. However, it is compounded by the desire

amongst specifiers, clothing manufacturers and end-users to

find a universal test method which simulates practice in all

respects. So far, this has proved to be a fruitless task.

2.1 Fundamentals

Diffusion of a substance through a layer is described by Fick’s

1st and 2nd Laws, which relate to the steady state and non-

steady state condition, respectively.11 Fick’s main hypothesis

states that the rate of transfer through unit area of a section,

F(x), is proportional to the concentration gradient measured

normal to the section, i.e.

F(x) = 2D(hc/hx) (1)

where c is the concentration of diffusing substance, x is the

space co-ordinate normal to the section and D is the diffusion

coefficient. The water vapour transmission (WVT) properties

of WBF membranes can be derived from eqn (1) using a

different proportionality constant (k) to relate WVT with the

pressure difference across the membrane surfaces (p1 2 p2) and

its thickness d.12

m/At = k(p1 2 p2)/d (2)

Many recognized breathability tests simply measure the

amount of water vapour (m) passing through a fixed area of

material (A) over time period (t) by gravimetry. The WVT rate

(m/At) obtained under steady state conditions has an SI

quantity of kg m22 s21, although the trivial unit g m22 day21

is most widely recognised by this industry.

2.2 Breathability testing: understanding the dilemma

One major concern with WVT testing is that results quoted do

not take the magnitude of the driving force, (p1 2 p2), into

consideration. More often than not, values for p1, p2 at each

surface of the membrane are guesstimated rather than

accurately measured. Hydrophilic membranes can show strong

and unusual dependences on applied relative humidity and

temperature, which determine p. These two factors alone make

comparing data obtained from different sources unreliable.

For instance, it is possible to generate apparent 40-fold

increases in WVT of WBFs solely by changing the set of

‘‘standard’’ test method conditions.8 During the WBF

technology boom of the 1980s, breathability testing was used

indiscriminately and often unscrupulously to claim higher and

higher WVT rates for marketing purposes. To a certain extent,

this one-upmanship mentality still remains.

An alternative approach is to visualise the membrane as a

barrier to water vapour diffusion, rather than a transmitter.12

In this case the reciprocal quantity of water vapour resistance

(WVR) is measured. The limiting WVR is zero implying that

the membrane is infinitely breathable, but of course this can

never be achieved with a barrier layer of finite thickness. There

are two main advantages in measuring WVRs: (a) the pressure

gradient is taken into account, and (b) the data are additive,

which is useful for assessing the overall resistance of real and

hypothetical clothing assemblies if the R values of individual

layers are already known.8 On the other hand, WVR is a more

difficult concept for consumers to grasp, and the various units

(e.g. mm still air, m2 Pa W21 and s m21) are illogical compared

with transmission rates.

Fig. 2 Schematic diagram of outer-garment constructions incorporating WBFs: (a) basic 2-layer fabric with coating on inside, water repellant-

treated outer surface; (b) 2-layer, coating on outside; (c) 2-layer plus loose liner fabric; (d) loose outer fabric plus scrim-supported membrane (drop

liner); (e) drop liner insert; (f) double membrane system, e.g. Dual ProtectionTM; (g) pictogram showing breathability and weatherproofness of a

3-layer laminate.

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WVR data are reliable for air-permeable membranes and

base fabrics, but less so for hydrophilic polymers, which are

strongly dependent on surrounding water vapour pressure or

water content, e.g. Fig. 3. True microporous components such

as (C) and (D) have low WVRs that barely change with

relative humidity (RH) at a given temperature.13 Conversely,

hydrophilic WBFs e.g. (A) are not especially breathable at low

humidities, but their WVRs decrease as the RH rises. The

shape and dip in the curve, especially in the 70–100% RH

range, depends on the hydrophilicity of the polymer, and

therefore on its own unique chemical composition. The net

outcome is that hydrophilic membranes respond to a build up

of water vapour inside a clothing assembly by swelling, thereby

becoming more breathable. This ‘‘sense and react’’ mechanism

can be classed an intelligent textile response,14 and similar to

the way human skin and biological polymers such as keratin

and cellulose behave. As might be expected, hybrid fabrics

containing both microporous and hydrophilic layers such as

type (B) show intermediate properties. Another feature in

Fig. 3 is that the ranking order observed at low–medium

humidities (A , B , C) is reversed at very high humidities,

and this exemplifies the problems with testing breathability

under different RH conditions.

Even today, the situation regarding breathability testing of

textiles has not been satisfactorily resolved. There are at least

three International Standards that may apply to WBFs, each

using different test conditions and expressing results in a

completely separate way.15 The ‘‘sweating hot-plate’’ method

or skin model (ISO 11092:1993) evolved from research into the

effects of clothing on human thermoregulation under varying

workloads, and yet the Standard test condition (35 uC;

RH 100% A 40%) is utterly meaningless for foul-weather

outer garments worn in real-life situations.

2.3 Breathability: demands versus reality

Thermoregulation involves a complex, interactive set of

physiological responses to balance metabolic heat production

(M) under workload (expended energy, Pw) with those heat

losses necessary to maintain a body core temperature (TC) of

approximately 37 uC.16 Fluctuations around the TC cannot be

tolerated for long before discomfort sets in, followed by

mental confusion, disorientation, serious illness and in extreme

cases, death.

Heat loss through the clothing occurs mainly by combined

dry processes (conduction, convection and radiation) and wet

processes (insensible perspiration and sweat), eqn (3).

M{Pwoutput

~ (HcondzHconvzHrad)dry loss

z

(HinsenszHsweatzHresp)wet loss

+ DSdebt

(3)

The relative importance of each heat loss component varies

depending on the workload and ambient atmosphere, the

individual’s metabolism and the clothing assembly worn. A

sedentary person has a typical heat output of about 100 W, a

respiratory heat loss (Hresp) of 25 W and a background water

vapour loss through the skin (Hinsens) of 250–300 g m22 day21,

equivalent to cooling of 12–15 W. Dry heat losses account for

the remainder (ca. 60 W), and the wearer usually remains

comfortable in light clothing (for example underwear, shirt

and trousers). Heat output under workloads, however, can be

as high as 1000–1250 W, i.e. similar to a one-bar electric fire,

but these levels of exertion can only be sustained for short

periods of time. Sweating is a particularly efficient cooling

mechanism in this case, because water has a very high latent

heat of evaporation (0.672 Wh g21 at 35 uC). This implies that

a sweat rate of 1 kg h21 provides 670 W cooling, but only if all

sweat evaporates away from the body. Clothing interferes by

trapping moist air, often resulting in condensation forming

somewhere within the textile layers. Some activities, e.g.

fire-fighting, produce even greater sweat rates, but coping

with 1 kg h21 is a reasonable target for moisture management

purposes.17

The heat debt, DS in eqn (3), is zero when heat output and

heat loss are in dynamic balance, i.e. the subject is in a state of

thermophysiological comfort. The case where sustained heat

output exceeds heat losses (+ve DS) or vice versa (2ve DS), has

potentially serious implications, which could lead to

hyperthermia or hypothermia, respectively. Someone under-

taking arduous exercise in foul-weather or cold climates may

encounter any one or all of these DS conditions during their

normal course of activity.

The mean WVT properties of a completely sealed survival

suit would have to be #10 000 g m22 day21 to satisfy a target

of Hsweat = 670 W, DS = 0, but this can only be achieved with

driving forces (p1 2 p2) in the order of several thousand Pa (by

eqn (2)).18 Such high pressure gradients are rarely achieved in

adverse foul/cold weather climates.

Fig. 4 shows water vapour pressure (p) as a function of

typical ambient temperatures and relative humidities, and can

be used to illustrate the difference between theoretical (skin

model, ISO 11092), standard textile test and real conditions.18

The isobars close up rapidly with decreasing temperature, but

even below 0 uC ice has a small and measurable p. This last

point is important, because in cold climates water can be

present in all three phases inside the clothing,19,20 and must be

dealt with to avoid chill effects. Most breathability tests are

carried out under isothermal conditions to avoid condensation

phenomena, and under high pressure gradients to allow short

Fig. 3 Schematic diagram showing water vapour resistance (WVR)

of WBF components with changing relative humidity (RH): (A)

hydrophilic coated fabric; (B) microporous coated fabric with

hydrophilic top coat; (C) pure microporous coated fabric; (D)

ePTFE membrane of 30 mm thickness.

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test periods. Neither test criterion simulates what happens in

practice for outer garments.18 For instance, the ISO test

methods15 operate respectively at 35, 23 and 20 uC, generating

nominal pressure gradients of 3093, 2168, and 818 Pa.

However, the WBF membrane is usually the next outermost

layer in a foul-weather clothing assembly, see Fig. 2, and its

operating temperature is close to ambient, not skin tempera-

ture (average 33 uC) as often mistakenly assumed. The actual

driving force for expelling perspiration (p1 2 p2) is therefore

considerably lower than for standard laboratory test condi-

tions. Temperate climates such as the UK have average

humidities in the range 70–95% and rainfall temperatures

of 2–8 uC, and the maximum pressure gradient across a

WBF membrane is only a few hundred Pa under these

conditions, Fig. 4. This means that optimistic target values

of 10 000+ g m22 day21 are unattainable, but through no

fault of the material. When (p1 2 p2) = 0, the membrane

appears to stop functioning, but it is actually in a state of

dynamic equilibrium where the vapour flows in each direction

are equal. WBFs must therefore be regarded as only one

component in moisture-management systems, where internal

clothing acting as instantaneous wicking layers (underwear) or

moisture buffer zones (mid-layers, piles, fleeces), and design

features that provide ventilation are equally, if not more,

important.5,18

WVR data afford a clearer comparison of WBFs with other

clothing layers than WVT rates, e.g. Fig. 5. The left-hand

dotted lines represent categories which relate to breathability

requirements for outer workwear garments.18,21 WBFs with

WVRs less than 20 m2 Pa W21 fall into the highest category,

Class 3 of EN 343 Specification,22 and many are equivalent to

normal clothing layers. In stark contrast, earlier types of

coated and wax-impregnated fabrics, Fig. 5(f) have much

higher resistances (200–1000 m2 Pa W21) and are notoriously

prone to condensation forming on their internal surfaces.3 The

right-hand dotted lines show further subdivisions (at 13 and

6 m2 Pa W21) used by the leading research organisation in this

field, Hohenstein Institute, to classify ordinary clothing layers.

Future revisions of the EN Standards may well adopt more

stringent pass levels for Class 3 materials, reflecting the

continual improvement in WBF performance.

3. Microporous membranes

Membranes with porous structures can be formed in numerous

ways, such as phase separation of unstable solutions, salt

extraction, mechanical frothing, and fibrillation of solid films.

The polymers employed include diverse polyurethanes (PUs),

acrylics, polyolefins and polyfluorocarbons, and these older

technologies have been extensively reviewed.23

A common feature of all microporous membranes is their

naturally dull, white appearance resulting from diffraction of

light through the surface cavities. The asymmetric structure,

Fig. 1(a), is made by the wet coagulation process developed in

the 1960s for PU synthetic leathers. As a general guideline,

the surface pores should be no larger than 1 mm to withstand

penetration by liquid water and the porosity should be as

high as possible to maximise WVT. Even so, the surface pores

can become contaminated with surfactants e.g. present in

washing powders, dry cleaning fluid, sun tan lotions, and skin

exudates which compromises their waterproof qualities. It is

now common practice to apply a thin, hydrophilic sealing

layer to a microporous membrane to overcome this potential

deficiency.8

Expanded poly(tetrafluoroethylene), ePTFE, is used in the

best-known WBFs, the Gore-TexTM laminates, with publicity

claims that there are more than 9 billion pores per square inch

of film. The ePTFE membrane, Fig. 6(a), is formed by biaxial

stretching and annealing at high temperature,24 and comprises

a 3-D network of predominantly crystalline nodes connected

by amorphous fibrils, typically with a porosity of 80% and

symmetrical pores of ca. 0.2–0.5 mm.

3.1 Still air layers and the air permeability mechanism

Normal textile structures, including tightly-woven fabrics such

as Fig. 6(b), have dead spaces between fibres and yarns

which are permeable to all constituents of air. Microporous

membranes also rely on the ability of water vapour to diffuse

through ‘‘still’’ air trapped in the vast labyrinthine structure.

Stabilised air layers act as protective heat buffers, because still

air has exceptionally low thermal conductivity (0.024 W mK21

at 20 uC) compared with water (0.6 W mK21), textile polymers

(0.1–0.5 W mK21) and other solid materials. The textile

Fig. 4 Variation of p with temperature and relative humidity.

Fig. 5 Typical WVR ratings for fabrics:21 (a) 2- and 3-layer ePTFE

laminates and uncoated microfibre; (b) microporous PU coated; (c)

hydrophilic PU coated and laminated; (d) low-swell PU system; (e)

non-breathable PU coated; (f) PVC, waxed, rubber etc. coated/

laminated; (g) underwear, light clothing layers; (h) heavy clothing

layers, fleece, pile etc.

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industry makes full use of this property for clothing and

bedding assemblies.25 ‘‘Warm’’ natural materials such as silk,

certain vegetable fibres (e.g. kapok), arctic animal furs, eider

and penguin down, have exceptionally high specific surface

areas (SSAs) that help stabilise air layers through viscoelastic

drag. Many creatures adapt to changing weather conditions by

varying the effective thickness of the surrounding air layer, for

example, by raising fur or puffing out feathers, and homo

sapiens can do the same by donning and doffing clothing

layers as required. Synthetic fibre developments that mimic

natural structures include hollow fibres, multilobal and

crenellated fibres, microfibre piles and fleeces and increasingly,

nanofibres.5 The internal structures of microporous mem-

branes also have large SSAs and possess this air-stabilising

characteristic.

It must be emphasised that still air is a poor transmitter of

water vapour as well as heat, which leads to a major comfort

paradox. Measures taken to improve the warmth of cold/foul

weather clothing by increasing still air-layer insulation will

automatically restrict breathability and increase the chance

of moisture build-up and condensation. The implications

are severe at low temperatures because sodden clothing

loses Hcond up to 25 times faster than dry material. Under

wind-free conditions, a naked human body is protected by

an air layer some 8–12 mm thick, which acts as a buffer to

heat and moisture vapour transfer with the environment.

Its effective thickness decreases exponentially with increasing

wind velocity, and this is one element of wind chill factor.

Modern breathable fabrics have equivalent still air thick-

nesses in a similar range (typically 4–10 mm),8 which puts

the high breathability of these materials into perspective

once more.

Fick’s Laws hold if there is no interaction between the

barrier layer and the diffusing substance, i.e. D is independent

of c in eqn (1).11 Diffusion of water vapour into still air is a

well-known Fickian process, where the diffusion coefficient

Dwa has an Arrhenius dependence on temperature (T, uC)

given by Dwa = (2.2 + 0.0147T) 6 1025 m2 s21. This

mechanism is easily demonstrated by measuring the WVR of n

stacked layers (n = 1–5) of hydrophobic, air-filled ePTFE

membranes under steady-state conditions (21 uC).12 The plot

of WVR versus effective film thickness is a straight line passing

through the origin, exactly as predicted by Fick’s 1st Law. The

WVR of a single ePTFE membrane (30 mm) is 0.25 Pa m2 W21

(equivalent to 6 s m21), one of the highest breathability ratings

of any commercial textile layer.

Increasing the polar or hydrophilic nature of the polymer

causes a progressive deviation from such ideal behaviour.

At the opposite end of the spectrum, hydrophilic swelling

membranes have D and d parameters which vary substantially

with c, and Fick’s Laws no longer apply.

4. Hydrophilic membranes based on PEO-PUs

Solid, hydrophilic membranes, Fig. 1(b), are impermeable to

air in textile terms. Instead, they employ a non-Fickian or

anomalous diffusion mechanism, involving transient hydrogen

bonding of water molecules to complementary functional

groups on the polymer chains. It is often described, textually

or pictorially, as a ‘‘molecular stepping-stone mechanism’’ on

swing tickets and in brochures for leisure and workwear

apparel, usually in conjunction with a pictogram such as

Fig. 2(g). Polar groups with hydrogen bond-acceptor sites such

as –OH, –NH2 and –CO2H can be utilised in hydrophilic

polymers.7,23 However, the ether linkage, –O–, with two sets of

lone pair electrons, has several advantages for commercial

breathable systems.26 The latter products are usually based on

copolymers of PEO with alternating ester (e.g. SympatexTM

membranes), amide (e.g. PebaxTM resins) or most commonly,

urethane (e.g. WitcoflexTM coating systems) segments in the

backbone. Interestingly, PEO is unique amongst the lower

polyether homologues in being water soluble, Table 1, which

indicates that hydrophilicity is not simply due to ether linkage

content (% w/w).

Other polyether segments commonly incorporated into PUs

via the corresponding glycols are poly(propylene oxide) (PPO)

and poly(tetramethylene oxide) (PTMEO). The value of n, the

average number of monomer units in the polyether segment,

typically varies from 6–40. Small proportions of PPO or

PTMEO segments are sometimes used in hydrophilic PUs to

reduce swelling,23 improve other properties such as softness

and elastic recovery or modify solubility behaviour. A route to

poly(trimethylene glycol)s from biorenewable raw material has

recently been patented,27 which opens up new possibilities for

PTMO-based PUs and other copolymers.

4.1 Basic composition

Linear PEO-PUs can be formed by the exothermic reaction of

diisocyanates with poly(ethylene glycol)s and difunctional

Fig. 6 Water-resistant structures: (a) SEM of ePTFE membrane

surface showing nodes and fibrils;7 (b) fluorochemical-treated micro-

fibre fabric made from TreviraTM polyester yarns with interstitial pores

of ca. 3 mm.

Table 1 Some properties of polyethers26

Chemical structure Acronym Water-solubleEther content(% w/w)

[CH2–O]n– PMO no 53.3[CH(CH3)–O]n– PAc no 36.4[CH2CH2–O]n– PEO yes 36.4[CH(CH3)CH2–O]n– PPO no 27.6[CH2CH2CH2–O]n– PTMO no 27.6[CH2CH2CH2CH2–O]n– PTMEO no 22.2

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chain extenders,28,29 as a melt, in various solvents or with

greater difficulty in water. The fundamental reaction is:

This is an idealized sequence where the PEO soft segments

are incorporated first, step (i), followed by chain extension of

the prepolymer with an equimolar proportion of diol (X = O)

or diamine (X = NH), step (ii). It predicts a perfect array of

alternating hard and soft segments along a molecular chain

where the number of repeat units, m, tends to infinity. In

practice, (a) the reaction stoichiometry is generally more

complicated than 2m : m : m, (b) more than three reactants are

often selected, (c) chain terminators are used to control Mw

and viscosity, and (d) the kinetics of steps (i) and (ii) affect

segment build-up. Consequently, the structure, length and

regularity of the segments, average chain length and therefore

the polyurethane molecular network as a whole are consider-

ably less than perfect.26 With certain diisocyanate–chain

extender (R1 : R2) combinations it is possible to elaborate

the hard segments first, i.e. carry out step (ii) before (i), which

can lead to improved physical properties,28 or use a one-shot

process where steps (i) and (ii) occur simultaneously. The terms

hard and soft correctly suggest that each segment type

contributes different properties in the final membrane.

Structure–property correlations for PEO-PUs have been

reported in the open literature, e.g. reference 30, but the

composition of commercial polymers is normally hidden

within complex patent specifications, or else remains a trade

secret. In order to achieve an acceptable balance between

breathability, waterproofness, swelling and adequate durabi-

lity, the overall PEO content and segment length are best kept

within certain limits, typically 40–60% w/w and n = 12–45.

PEO-PUs with parameters outside this range can be gainfully

used in other, non-WBF applications.

4.2 Some practical aspects of the textile technology

Eqn (2) implies that hydrophilic membranes should be as

thin as possible to maximize WVT, but this is inevitably

limited by manufacturing tolerances, handling considerations

and the conflicting durability and cleaning requirements.

Uniform films for transfer coating and lamination are typically

1.5–2.0 m wide and 15–25 mm thick, although 5–10 mm

PEO-ester films produced by biaxial stretching have appeared

on the market.

By comparison, PEO-PU coatings applied directly onto the

substrate are non-uniform in thickness, because they follow the

surface contours of the base fabric and fill into the interstices

between yarns. Direct coatings have a stratified composition,

Fig. 2(a), and surprisingly complex chemistry. They may also

contain additives such as pigments, flame retardants, slip aids

and matting agents in one or more layers. Base and middle

coat PUs are normally very soft and extensible, and require

crosslinkers to afford adequate ‘‘wet’’ properties, durability

and adhesion promotion to the fabric substrate.26 They are

often based on TDI (commercial blends of 2,4- and 2,6-

diisocyanatotoluene) with PEO contents higher than 50% w/w

and supplied as 45–55% solids in MEK (methyl ethyl ketone)

or blends. Top coats are harder, more abrasion resistant and

less hydrophilic; they are usually manufactured from MDI

(4,49-diisocyanatodiphenylmethane) with lower PEO contents

than base coats and require more powerful solvents (usually

supplied at 30% solids in DMF blends). Crosslinkers are

normally avoided in top coat formulations, because the outer

surface must remain thermoplastic for taping of seams in

finished garments. As each layer is progressively built up, the

solvent is removed and the dried layer becomes the substrate

for the next. This is best carried out in a one-pass operation

and lines with 2, 3 or even 4 coating heads and associated

ovens in tandem are now commonplace. The final consolida-

tion process is usually carried out at higher temperatures to

ensure complete removal of solvents, and crosslinking of the

base-coat polymer before the coated fabric is rolled up.

In contrast to the porous types, hydrophilic coatings are

transparent and glossy, and top coats are usually matted to

give an acceptable surface lustre. Ironically, some coaters

incorporate white pigments into a hydrophilic coating so that

it resembles a microporous system, or perhaps can be passed

off as one in the WBF marketplace. By doing so, high loadings

of pigment can detract from the excellent physical integrity of

the hydrophilic PU, which was one of the main reasons for

their development in the first instance.

4.3 Phase separation and swelling effects in membranes

As the coating or film consolidates, the segmented molecules

undergo phase separation to a lesser or greater extent. It is

initiated by hard segments agglomerating together via intra-

and inter-molecular hydrogen bonding involving urethane

and/or urea groups, Fig. 7.

The excellent physical properties (tensile strength, elonga-

tion, elastic recovery, tear strength etc.) of many PUs rely on

optimal phase separation. This is best achieved using R1, R2

Fig. 7 Phase separation in hydrophilic PEO-PUs:18,26 (a) SEM of

non-porous membrane surface etched with DMF vapour showing hard

and soft contours; (b) highly schematic 2-D diagram of nanophase

structure; (c) ‘‘stick and string model’’ of segmented molecular chain.

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residues that form linear, compact or symmetrical hard blocks,

which are capable of packing closer together than those

obtained with asymmetrical species. For this reason, MDI-

based polymers tend to have improved physical properties

compared with TDI analogues. In hydrophilic PUs, the hard

segments can be visualised as forming a constraining mesh

around the PEO soft segments, which prevents excessive

swelling or even complete dissolution of the polymer in water.

Crosslinking with e.g. multifunctional isocyanates leads to

covalent bonds forming between adjacent hard segments.26

At higher water vapour pressures, PEO-PU membranes

absorb increasing numbers of water molecules and start to

swell. Condensing water can also be slowly absorbed by the

PEO-rich surface of the polymer membrane, which is an

important and often overlooked advantage over hydrophobic,

microporous types. Bringing liquid water into contact with

the membrane can increase the WVT rate substantially, by a

factor of 3 or more, over the dry state.12 Swelling is therefore

an intrinsic property of these materials,26 and vital for their

modus operandi. It has to be encouraged and yet controlled,

through correct choice of the hard segment constraint (R1, R2),

hard : soft segment ratio and optional crosslinking system.

Base-coat and top-coat polymers typically absorb 70–100%

and 40–70% w/w, respectively when immersed in water at

room temperatures.21

4.4 Effect of temperature on hydrophilicity

PEO homopolymer, PEGs, PEG ethers and other PEO-

containing species show inverse water solubility features with

temperature. This anomaly is also observed when measuring

the WVR or, more conveniently, the swelling properties of

PEO-PU membranes over a range of temperatures.18

For example, Fig. 8 shows the effect of varying PEO content

and chain length on equilibrium water uptake of a series of

PUs (A–D) with constant hard : soft segment ratio (1.66), as

detailed in Table 2. All films clearly become less hydrophilic

with increasing temperature, but the effect is more pronounced

as the size of the PEO nanophase increases (i.e. film A & D)

and becomes more difficult to constrain. Because water is

absorbed exclusively by the soft segment nanophase, the

results can also be expressed as the hypothetical number of

water molecules associating with each ether oxygen atom

(H2O : EO ratio) at a given temperature.31 This provides a

comparative measure of hydrophilicity. Other researchers have

shown that PEO-PUs and PEO itself form stable complexes

with water at H2O : EO ratios of 1 and 3.32–34 Above the

trihydrate level, water molecules in PEO-PUs start to combine

and form clusters within the molecular network,31 which is

detrimental for vapour transport through WBFs.18 Empirical

studies in Baxenden laboratories have also shown that PEO-

PUs with H2O : EO ratios of 2–4 at 20 ¡ 2 uC prove most

suitable for textile membranes.35 Commercial products are

often selected so that an optimum transport condition of

H2O : EO # 3 occurs at typical foul-weather temperatures.18

Film C could fall into this category. Alternative polymers with

ratios below 2 are insufficiently breathable, e.g. D, whilst those

with higher ratios swell too much for practical purposes, B. At

even higher ratios of 7+, such as A, there is an onset of

hydrogel properties where water is retained rather than

transmitted. In the extreme case where H2O : EO tends to

infinity, the polymer becomes water soluble.

4.5 Elucidating the water vapour transport mechanism

During the past decade, fundamental studies have been carried

out on the biologically important PEO molecule that provide

an insight into how hydrophilic PU membranes actually work

at the nano-level. Vapour diffusion through hydrophilic PU

membranes occurs predominantly through the interconnected

PEO soft segments.

Water molecules have a greater hydrogen-bond affinity for

the EO ether oxygen (34–38 kJ mol21) than they have for other

water molecules in clusters (19–23 kJ mol21),36 which may

assist initial absorption into the surface monolayers and

transfer through the membrane under a concentration

gradient. It is now widely accepted that the thermodynamically

preferred conformation of PEO chains (n . 10) is a 72 helix,

formed by a repeating trans–gauche–trans (tgt) arrangement of

C–C–O–C bonds in the backbone,36–38 Fig. 9(a).

There is a striking similarity between the adjacent and next

nearest O–O distances in the 72 PEO helix and those in water

clusters,39 Fig. 9(c). This exceptional stereo-compatibility is

held responsible for the water solubility characteristics of PEO

and many derivatives, and it almost certainly accounts for the

high permeability of PEO-PU textile membranes.18,21 There is

strong supporting evidence for this conjecture. First, the

closest polyether homologues, PMO and PTMO, have

incompatible O–O arrangements (being 0.22 nm and 0.48 nm

apart, respectively) and are not water soluble or hydrophilic.

Secondly, replacing PEO with equivalent PTMEO orFig. 8 Equilibrium water uptake of homologous PEO-PU films.

Table 2 Hydrophilic properties of PEO-PU films A–D (see Fig. 8)a

Film

PEO nanophase H2O : EO ratio at

(% w/w) av. n value 2 uC 35 uC 70 uC

A 72 44 7.29 5.73 3.11B 66 33 4.97 3.81 2.14C 54 22 2.90 2.09 1.07D 44 13 1.18 0.83 0.55a Polymers A–D were prepared from MDI, PEG (of Mw 2000, 1500,1000 and 600 respectively) and butan-1,4-diol (molar ratio 2 : 0.8 :1.2) in a one-shot, 100% solids process, then dissolved in DMF andcast into films.

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especially, isomeric PPO segments drastically reduces the

breathability properties of poly(ether-urethane)s.29 Thirdly,

water molecules are thought to form bridges between

adjacent ether oxygens, which help stabilise the helix.40

Empirical and theoretical studies have shown that stretching

a PEO strand in the presence of water alters the spatial

O–O distances, destroys the bridging capability and progres-

sively converts the hydrophilic tgt helix into a relatively

hydrophobic, planar ttt conformation.40–42 The general feature

that PEO species, including the polyurethanes in Fig. 8,

become less hydrophilic with increasing temperature can then

be explained in terms of growing imperfections (random

gauche defects) in the 72 helix, brought about by increased

thermal motion.

The 72 unit can be visualised as a grooved cylinder,36 with its

inside electronegative relative to the outside, due to the

location of the ether lone pair electrons, Fig. 9(d). The

attractive theory that PEO helices might behave as bundles of

nano-capillaries, with water molecules temporarily bonding to

the internal walls, can be discounted because the central

channel is too narrow. A more likely scenario is that water

molecules bond to the outside of the PEO chains, possibly

using the helical groove to form a hydration sheath.36 Taking

the favoured case where H2O : EO # 3, there are only two

hydrogen-bond acceptor sites per ether oxygen, so a third

water molecule must somehow bridge across two already

bound molecules. It is possible to draw several structures for

inter- or intra-chain bridging involving three water molecules

per EO unit, any of which could easily be integrated into a

‘‘stepping-stone’’ mechanism.35

4.6 Low-swell developments

In most garment constructions, the breathable membrane is

located on the inside, partly to protect it from snagging and

abrasion damage whilst the outer fabric surface is treated with

durable water repellants to help shed rainfall, Fig. 2(a),(g).

However, there are other potential applications, for instance in

some types of mattress covers, medical dressings, shoes and

easy-wipe workwear, where the membrane normally forms

the outside layer, Fig. 2(b). Conventional swelling polymers

cannot be used in this case, because they often develop

unsightly wrinkling or water-spotting effects in wet or rainy

conditions, or else are too fragile in the swollen state.

Several approaches have been taken to solve this problem,

but all lead to the inevitable trade-off between dimensional

swelling and breathability.23 Most attention has focused on

modifying the PEO soft segment, for example, by using ABA

or (AB)n copolymers which combine shorter PEO units with

PPO or PTMEO units.

More recently, poly(urethane-urea)s with exceptionally low

water uptakes (typically , % w/w), no perceptible swelling

and acceptable WVRs at lower coating weights have been

introduced for some niche markets. These low-swell polymers

have substantial ether atom contents (15–20% w/w) and are

therefore water vapour permeable,43 yet they are PEO-free and

not hydrophilic in the accepted sense.7 Fig. 10 suitably

illustrates where they fit into the breathability spectrum of

solid polyurethanes used by the textile industries.21 The wedges

were constructed by measuring the WVRs of stacked layers of

appropriate films,35,43 using the modified ISO 15496 test

method previously described.12 By combining low-swell and

hydrophilic chemistries, e.g. using physical blends of the

separate polymers or preparing copolymers with two phase-

separable soft segments, it is possible to develop membranes

with an intermediate balance of properties, Fig. 10(c).

Composite membranes, e.g. tri-layers of polymers with low,

medium and high swell propensity, show interesting aniso-

tropic behaviour,43 with WVT/R properties depending on

the direction of water vapour flow and stacking order. This

promises to be an exciting area for further expansion of

breathable polyurethane technology.

4.7 Crossovers with other technologies

PEO-PUs are already used in, or may be considered for, many

other applications, see Fig. 11, which is by no means an

exhaustive list. These products benefit from one or more of the

following properties, imparted by the PEO segment.

1. Low water content: the moisture regain of PEO-PUs is low

(ca. 1–2%), but still significantly higher than that of non-

breathable PUs. This provides anti-static properties,44 which

Fig. 9 (a) Helical structure of PEO molecule with n = 20;36 (b) 72

helix dimensions (7 EO monomer units per 2 complete turns); (c)

distances between oxygen atoms; (d) cross section through helix.

Fig. 10 Breathability wedges for textile-grade PU membranes:21 (a)

typical non-breathable (water uptake ca. 1–3% w/w); (b) low swell

(1–3%); (c) hybrids of low swell and hydrophilic (10–25%); (d)

hydrophilic top coat (40–70%); (e) hydrophilic base coat (70–100%).

The vertical line represents a nominal 25 mm thickness.

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can be further improved by incorporating carbon black, metal

powders and organic conductors.

2. Water transport: vapour diffusion has been discussed in

detail, but PEO-PUs also have surface energies that encourage

wetting out and wicking of liquid water.5,18

3. Swelling capacity: these materials behave as hydrogels at

high PEO contents, n values and H2O : EO ratios.33 The 3-D

molecular network can be designed so that the degree of

swelling can be precisely and reproducibly controlled, which is

essential for e.g. drug delivery systems.46

4. Crystallinity: this increases with the size of the PEO phase,

i.e. with chain length or n value.47 Highly crystalline, radial

spherulites may be observed in the soft segments with PEO n

values .60.46 The degree of soft phase crystallinity influences

gas and vapour diffusion, physical properties such as modulus

and tear strength, and the effective glass transition tempera-

ture, Tg, of the polymer. PEO-PUs with relatively high Tgs

(above 10 uC), e.g. DiaplexTM breathable membranes, are

claimed to have WVT properties that change significantly and

advantageously with ambient temperature.48,49 However, their

effectiveness has also been refuted.50

5. Bio- and haemo-compatibility: PEO-rich surfaces are

resistant to protein absorption and blood platelet attachment.

Pendant PEO groups are particularly effective.51

6. Lewis basicity: to a certain extent, PEO segments can be

regarded as open-chain versions of crown ethers, such as

18-crown-6, with chelating and catalytic activity. They can use

the electron donor properties of the oxygen atoms to bind

small metal ions, e.g. Li+ for polymer electrolytes,52 and other

electropositive species.

5. Conclusions

Breathable polyurethanes are an important class of

polymers widely used, for example, in sportswear, weather-

resistant workwear and medical textiles. There are two basic

types—microporous and hydrophilic membranes—which

transfer water vapour by markedly different mechanisms.

Like ordinary fabrics, microporous membranes rely on

having a degree of air permeability. Water vapour and other

constituents of air can freely diffuse through the intercon-

nected holes and passageways under a suitable concentration

gradient. This transport mechanism closely obeys Fick’s Laws

of Diffusion, and increases with increasing temperature and/or

relative humidity gradient. The ultimate waterproof properties

of microporous membranes such as Fig. 1(a) and 6(a) depend

on the maximum size of the surface pores and their ability to

withstand applied water pressure (commonly known as

hydrostatic head resistance, HHR). Agencies which lower the

surface tension of the liquid water, and therefore reduce

the contact angle that it makes with the pore walls, will lower

the HHR. This has been observed in practice with washing

detergents, dry cleaning fluids, sun-creams and other products

containing surfactants.

Conversely, hydrophilic polymers with solid (non-porous)

structures, e.g. Fig. 1(b) and 7(a), give much higher HHRs and

are intrinsically more waterproof than microporous types.

They also have zero air permeability by textile standards.

Instead, they breathe via an absorption–diffusion–desorption

mechanism which is accelerated by specific water-binding

components, usually PEO segments, incorporated into the

molecular chains. Another common feature of hydrophilic

membranes is that they undergo controlled swelling at high

vapour pressures or when in contact with liquid water. In the

swollen state, the water vapour transfer properties are

enhanced, typically by a factor of three. This process shows

abnormal dependences on temperature and relative humidity,

and is often described as anomalous or non-Fickian diffusion.

Many tens of millions of garments are manufactured each

year incorporating PEO-PU films and coatings as a barrier

layer. These products have evolved so that a good balance

of breathability, waterproofness, durability, laundering and

aesthetic qualities can be achieved at relatively modest cost.

However, because polyurethane chemistry is so infinitely

variable, the polymers outlined in section 4.1 form only a

small fragment of the technology available—plenty of scope

remains for future developments and transfer into new

application areas.

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