Keeping dry in the rain - European-coatings.com

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Quelle/Publication: European Coatings Journal 

Ausgabe/Issue: 09/2009 

Seite/Page: 1 

Keeping dry in the rain 

Hydrophilic polyurethane (HPU) membranes are 

widely used in the construction of various types 

of ‘breathable’ fabrics. The history, formulation, 

application procedures and end-uses of these materials 

are outlined. In addition to many different forms of 

functional clothing, HPU systems are used in a growing 

range of medical applications. 

Hydrophilic textile treatments enhance user comfort 

Robert Lomax 

As the name suggests, hydrophilic polymers have an 

unusual affinity for water. In the case of polyurethanes, 

this hydrophilic property spectrum ranges from polymers 

that are completely water-soluble to those with surfaces 

that merely allow better wicking and wetting. Hydrophilic 

polyurethanes (HPUs) used in the textile coating and 

laminating industry fall somewhere in between these two 

extremes. 

HPU membranes have a solid structure (typically 1530 

µm thick) which prevents ingress of liquid water, but 

transmits moisture vapour as individual water molecules 

via an enhanced diffusion mechanism [1]. They are 

easily distinguished from alternative breathable membranes 

(made from PUs, PTFE or polyolefins) which have a 

permanent microporous structure as shown in Figure 1. 

Not surprisingly, HPUs are widely used in waterproof, 

breathable fabrics (WBFs). These products offer full 

weather protection, with the ability to allow perspiration to 

escape through the outer layer. Consequently, garments 

made from WBFs are less prone to condensation, and 

can provide a real thermophysiological benefit, dependent 

on the wearer’s activity level and the ambient weather 

conditions. 

Development of HPUs commenced in 1970s 

Our HPU technology stems from extensive polymer 

research carried out at the Shirley Institute, Manchester, 

UK, in the late 1970s and 1980s [2]. It concentrated on three 

main areas: 

››› Weatherproof clothing. This research was originally 

carried out for the UK Ministry of Defence. The aim was 

to meet all the existing requirements for PU-coated military 

rainwear plus adequate breathability (initially, a moisture 

vapour transmission rate of 

2000 g/m2/day was set). Once this objective was achieved, 

research switched to the emerging civilian market for WBFs, 

which had been stimulated by the introduction of "Gore-Tex" 

laminates. 

››› Mattress covers. Direct and transfer-coated fabrics for 

hospital and other institutional mattress covers, where the 

PU layer forms the exposed outer surface. 

››› Other medical textiles. Subjects included infection 

barriers (such as occlusive wound dressings, hospital 

drapes, masks and surgeons’ gowns), semi-disposable 

workwear, gloves, bio-separation membranes, hydrogels, 

foams, moisture vapour permeable adhesives, anti-static 

coatings and even nappy liners with a mixed degree of 

success. 

These remain amongst the main applications for HPUs in 

today’s textile industry, which are summarised in Table 1. 

How HPUs are formulated 

HPU coatings and films can be produced from organic 

solutions, from aqueous solutions or dispersions, and from 

various 100 % solids systems. The majority incorporate 

poly(ethylene oxide) PEO-segments within the urethane 

polymer backbone. PEO itself is water-soluble, and in 

copolymers such as HPUs and certain polyamides and 

polyesters it functions as the vapour transfer agent. 

It is important to note that this "stepping-stone" type of 

mechanism for water transfer is stereo-specific for PEO and 

water molecules, and that other polyethers do not show 

hydrophilic characteristics [1]. The fundamental chemistry 

is shown in Figure 2. 

Polymer chemists will recognise that this sequence is rather 

simplistic, and that in practice the chemistry is far more 

complex in terms of the type, number and molar ratio 

of constituents. It also allows for great versatility in the 

design of HPUs for specific application techniques and 

performance requirements. 

The urethane (X = O) or urea (X = NH) hard segments 

function as molecular constraints, which control the 

vapour transport and liquid water uptake properties of the 

membrane. The residues R1 and R2 are derived from 

common diisocyanates, diols and diamines used in the PU 

industry, and it is the elaboration of these hard segments 

which is most often the inventive step in HPU technology. 

As a guideline for achieving hydrophilicity, the overall PEO 

content and segment length are best kept within certain 

limits, typically 3060 % w/w and n values (the average 

number of EO units per segment) of between 12 and 

45. For the interested reader, further details on HPU and 

microporous membrane formation are given in Träubel’s 

excellent book [3]. 

Fabric structure may take many forms 

The location of the HPU layer within a garment assembly 

is important for moisture management, durability and 

general aesthetics. Some of the main variations are shown 

schematically in Figure 3. The earliest garments were made 

from direct-coated fabric usually a plain-woven polyamide, 

weighing 60 or 120 g/m2 with the membrane on the inside, 

Figure 3(b), to minimise snagging, puncture or abrasion 

damage. 

Liner fabrics were often omitted to reduce costs, which 

meant that the membrane could come into direct contact 

with the skin. When this happened, the wearer sometimes 

complained of clamminess or a ‘wet-cling’ sensation. 

Design of garments quickly evolved so that bonded 

and loose liners, drop liners and inserts, Figure 3 (dh), 

improved the handle, tactile qualities and moisturedissipating 

properties of the original unlined construction. 

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Simple liners based on lightweight scrims or tricots can 

be replaced by waddings or other bulky textiles for added 

insulation in garments that combine foul- and cold-weather 

protection. 

HPUs can be applied to textiles by the well-established 

techniques of direct coating, transfer coating (using 

siliconised belts or release papers) and film lamination. 

WBFs, for example, are normally made in continuous rolls, 

with lengths varying between 100 and 5000 m and widths of 

1.5 1.8 m for apparel. Wider machinery (up to 2.23.0 m) is 

required for certain end-uses such as tarpaulins, mattress 

covers and horse blankets. 

Performance requirements summarised 

The coating weight for WBFs varies according to the fabric, 

but is typically 2025 g/m2 for lightweight synthetic fabrics 

and up to 5070 g/m2 for heavier synthetic, cotton, cotton/ 

polyester and textured fabrics such as Cordura. This usually 

represents an effective barrier layer thickness of 2030 µm, 

an optimum for a good balance of water vapour transfer 

properties, liquid water resistance and general durability. 

A respected performance benchmark is EN 343:2003, 

which "specifies requirements and test methods applicable 

to materials and seams of protective clothing against the 

influence of precipitation (e.g. rain, snowflakes), fog and 

ground humidity". It classifies three grades of fabric, where 

the technical balance of properties improves progressively 

from Class 1 (effectively non-breathable) through to Class 

3, as shown in Table 2. 

A good HPU-coated fabric or laminate should easily satisfy 

Class 3 requirements. Our original HPUs were prepared 

in solvent for coating directly onto the surface of closelywoven 

filament nylon and polyester fabrics using knife-overroll 

and knife-on-air techniques. This remains a major outlet 

for HPU, and it can be used to illustrate WBF manufacture. 

Multilayer coatings provide optimum performance 

Solvent-based HPUs are typically made in 1015 tonne 

batches, and resemble golden syrup in appearance and 

‘spreading’ consistency. As a broad guideline, 1 tonne of 

HPU solution will coat 10,000 running metres of mediumweight 

polyester fabric, sufficient to make about 7000 

average-size anoraks. 

Direct coatings are non-uniform in thickness, because the 

applied compound follows the surface contours of the fabric 

and fills in the interstices between yarns. They usually have 

a stratified composition, as in Figure 1(f), with each layer 

having a different chemistry and fulfilling a different function. 

For instance: 

››› Base coats are normally very soft and extensible, to 

minimise the natural stiffening effect on fabric handle and 

aesthetics. Crosslinkers are required to provide adequate 

"wet" properties in use, durability and chemical bonding 

as well as mechanical adhesion to the base substrate. 

They are normally supplied as 4555 % solids in methyl 

ethyl ketone (MEK) or ethyl acetate, with solution viscosities 

of 200-400 poise. Multifunctional isocyanate or MF resin 

crosslinkers, diluents and optional additives such as 

pigments and flame retardants can be mixed in just prior to 

use. 

››› Middle coat(s) often have a similar chemistry to base 

coats, and are usually applied as filling layers to increase 

the weight and thickness of the protective layer. Skimping 

on coating weight is often false economy, especially with 

the more uneven substrates. Unless surface protrusions 

are completely encased by the polymer coating, they 

remain weak spots and the coated fabric is more prone to 

hydrostatic failure in use. 

››› Top coat HPUs are tougher, more abrasion resistant 

and usually less hydrophilic than other layers. The polymers 

show strong intermolecular hydrogen bonding, and require 

more powerful solvents such as dimethylformamide (DMF) 

or DMF blends to produce coating solutions (typically at 

2530 % solids). 

Top coats usually contain finely-divided silica to reduce 

gloss. Crosslinkers are not required, because the top coat 

must remain thermoplastic and suitable for hot-air taping. 

This is a technique for applying narrow heat-sealing tapes 

over the stitching inside garments to make the seams fully 

waterproof. 

As each layer is progressively built up, the solvent is 

removed and the dried layer becomes the substrate for the 

next. The best practice is to carry this out in a one-pass 

operation and lines with 2, 3 and even 4 coating heads and 

associated ovens in tandem are now commonplace. 

These machine lines can be up to 100 m long, operating 

at speeds of 15 40 m/min. The coating as a whole must be 

fully consolidated at the end of the operation, with all layers 

working in unison. This requires close control of successive 

drying and curing cycles, so that all solvents have been 

removed and adequate crosslinking has taken place before 

the coated fabric is finally wound up. 

Transfer coating is useful on ‘difficult’ fabrics 

Similar solvent-borne HPUs can be used for transfer 

coating, where the HPU layers are laid down in reverse 

order, i.e. top-middle-base. The first (top coat) layer is 

applied onto a continuous roll of siliconised release paper, 

which takes all the applied machine tensions. This top coat 

is dried and subsequent coat(s) are applied. 

Whilst the final base (or tie) coat is still tacky, the fabric 

substrate is brought into contact with it under a nip roll. 

The film-fabric-paper laminate then passes through the final 

drying/curing oven and onto the winding station, where the 

coated fabric and paper are stripped apart and separately 

wound up. 

This technique allows open, flimsy and dimensionally 

unstable fabrics such as coarse weaves, knits, nonwovens 

and scrims to be used as carriers. Release papers can be 

re-used a number of times, but they make the process more 

costly than direct coating. 

Transfer coating machinery can also be used to produce 

uniformly thin HPU membranes which are stripped from the 

paper and wound up as a free-standing film, or interleaved 

with a release carrier such as polythene sheet. Cast-film 

production has some advantages, e.g. for small production 

runs incorporating a variety of pigments, biocides, special 

effect additives etc. The main applications for cast HPU film 

are customised WBFs and medical textiles. 

Solvent emissions can be reduced in various ways 

Environmental legislation and the recurring threat to 

coating companies using large volumes of solvent-based 

hydrophilic PU prompted the development of water-based 

systems in the early 1990s. They have some drawbacks, 

most of which arise from the physical properties of water 

itself. Nevertheless, aqueous HPUs are successfully used 

by a number of coaters in the textile industry. 


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100 % solids systems are hydrophilic versions of 

thermoplastic PUs (HTPUs), manufactured by a solventfree 

process following the basic chemistry shown in Figure 

2. They are used in two ways by the industry. HTPU 

pellets can be dissolved and converted into homogeneous 

coating solutions, typically 2530 % solids in DMF or other 

strong solvents. These solutions can then be used for 

normal direct and transfer coating operations, as outlined 

above. Conversely, HTPUs can be thermally reprocessed 

above their melting points (ca. 180190 °C) for calendering/ 

extrusion coating onto fabrics and for blown film extrusion. 

Reactive PU hot-melt adhesives are special types of HTPU 

characterised by relatively low molecular weights, low 

melting points and free isocyanate contents of ca. 35 %. 

They are designed as one-component adhesives applied 

in the molten state, typically at temperatures of 90140 °C, 

using a screen printing unit, slot nozzles or more usually a 

heated gravure-roll coater. 

Once on the substrate, the polymer layer absorbs 

atmospheric moisture, which initiates a series of irreversible 

chain extension, adhesion promotion and crosslinking 

reactions within the PU molecular network. These products 

have extremely good solvent, wash and dry cleaning 

resistance compared with other textile adhesives. 

This is a relatively new, but growing, area for HPUs 

in textiles. The main interest to date is for laminating 

breathable membranes, especially ePTFE, to a fabric or 

fabrics. Using a continuous, breathable adhesive layer 

maintains the high water vapour transport of the membrane, 

and at the same time optimises coverage and bond 

strength. 

REFERENCES 

[ [1] Lomax G. R., J. Mater. Chem., 2007, 17, 2775-2784 

[2] Lomax G. R., J. Coated Fabrics, 1985, 15, 4066 [3] 

Träubel H., New Materials Permeable to Water Vapor, 

SpringerVerlag, Berlin, 1999, part 4, pp.107-213 [4] 

Baxenden Chemicals Ltd., World Patent 2006 075 144 

Results at a glance 

Hydrophilic polyurethane (HPU) membranes are widely 

used in the construction of various types of ‘breathable’ 

fabrics. Unlike other breathable systems, they do not 

contain physical micropores but rely on the hydrophilic 

properties of polyethylene oxide within the polymer to 

transport moisture. A number of different multilayer fabric 

constructions are used to enhance wearer comfort. The 

coatings themselves are usually multilayer, to optimise 

bonding to the fabric (base coat) ensure full coverage and 

effective moisture transport (mid coat) and provide abrasion 

resistance (top coat). HPU systems are used not only 

in many forms of functional clothing, but also in medical 

applications ranging from wound dressings to mattress 

covers. When the membrane forms the outer, visible layer 

of a material, it may be necessary to restrict the (reversible) 

swelling of the HPU which occurs during moisture pick-up. 

Water swelling sometimes has to be minimised 

HPUs transport moisture vapour by a mechanism that 

encourages its rapid absorption into the polymer. At high 

humidities, and especially when liquid water is in contact 

with the polymer, this causes the HPU layer to swell. Once 

the moisture is removed, the polymer returns to its normal 

state. 

For most end-uses, this layer is hidden from view and the 

reversible swelling mechanism goes unnoticed. However, 

in some important applications such as mattress covers, 

medical dressings, shoes and easy-wipe workwear, the 

membrane forms the visible outer layer, as in Figure 3(c). 

Normal HPUs are rarely used here, because they may 

develop unsightly wrinkling or water-spotting effects in wet 

or rainy conditions. 

Most approaches to solving this problem lead to an 

inevitable trade-off between breathability and dimensional 

swelling. Baxenden has developed a top coat system 

based on PEO-free PUs that have exceptionally low water 

uptake (< 2 % w/w) yet reasonable water vapour transfer 

properties when they form part of an integrated coating 

system. 

They fit into a spectrum of products where breathability 

and swelling can be traded off to suit a particular 

application, depending on membrane thickness, see Figure 

4. Composite membranes, e.g. trilayers of low, medium 

and high swell propensity, show interesting anisotropic 

behaviour where breathability depends on the direction of 

water vapour flow and the stacking order of the individual 

layers [4]. This is a development area that complements the 

company’s existing HPU technology. 


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Figure 1: Some features of membranes used in waterproof breathable fabrics 

(WBFs). Top row: surfaces of membranes; bottom row, sections through 

coatings, laminated to polyamide fabric unless otherwise stated. Left to right: 

Solid hydrophilic coating (a) showing multiple layers at (f); Microporous PU 

produced by wet coagulation (b) and (g); Microporous PU produced by phase 

separation (c) with very thin pore-sealing topcoat shown at (h); Mechanicallyfoamed 

PU dispersion (d) with mechanically-foamed acrylic coating shown in 

(i) with first layer crushed; ePTFE membrane (e) and PU film produced by salt 

extraction laminated to polyester with discontinuous adhesive (shown as solid 

dots) (j) 


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Figure 2: Basic reaction sequence for 

manufacturing HPUs based on PEO 


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Figure 3: Schematic diagram of outer 

garment constructions incorporating 

WBFs: (a) membrane only; (b) basic 

2layer fabric with coating on inside, 

water repellenttreated outer surface; 

(c) basic 2layer fabric with coating 

on outside; (d) 2layer plus loose liner 

fabric; (e) typical pictogram showing 

breathability and weatherproofness 

of a 3layer laminate (often used in 

advertising, brochures and swing 

tickets); (f) loose outer fabric plus 

scrimsupported membrane (drop 

liner); (g) drop liner insert; (h) double 

membrane system, e.g. "Dual 

Protection" using ‘cold bridging’ 

principle 


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Figure 4: Breathability wedges 

for textilegrade HPU membranes 

(showing water uptake w/w): (a) 

nonbreathable (13 %); (b) low-swell 

development, top-coat (13 %); (c) 

hybrids of low swell/hydrophilic (1025 

%); (d) hydrophilic top coat layer (4070 

%); (e) hydrophilic base coat layer 

(70100 %)

Keeping dry in the rain - European-coatings.com

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