Empa Activities 09/10
Adaptive breathable membranes
Materials meet Life
Monolithic breathable membranes are key elements in the production of advanced sports apparel. We have developed waterproof polymeric membranes with excellent moisture transport properties, the latest being adaptive to different climatic conditions.
Daniel Crespy, Romain Simon, Ana Maria Popa, René Rossi
The clear influence of thermo-physiological comfort on performance has driven the sports clothing industry towards the development of different systems with better moisture management and improved thermal transfer, as well as the adaptation of these two properties to environmental conditions. The main products designed to fulfill these requirements are waterproof breathable membranes. These are usually polymeric sheets which are hydrophobic enough to prevent the permeation of water droplets through the clothing while allowing the transport and subsequent evaporation of sweat. Most systems existing on the market, while being extremely performing – displaying water vapor transmission rates (WVTR) in the order of kg/m 2 day – have low or no adaptivity to environmental conditions.
23.2 µm
Fig.1: Low magnification SEM image of a vertical crosssection through a monolithic EAA-mPEG membrane.
1500 Membrane A Membrane B EAA-mPEG membrane
1300 WVTR [g/m2.24h]
Fig. 2: WVTR in relative humidity conditions for the EAA-mPEG membrane and two commercially available membranes respectively. Membrane A is a microporous membrane without hydrophilic coating, whereas membrane B is a monolithic membrane.
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After purification and dissolution in a suitable solvent, membranes with thicknesses as low as 25 mm were fabricated through bar coating. A low magnification SEM image of such a membrane is presented in Figure 1. An important requirement for the final product is the maintaining of a high enough breathability without compromising the waterproofness, or hydrolysis resistance necessary for most clothing-oriented applications. The optimum hydrophilic-hydrophobic balance was achieved for a film comprising 50% (w/w) EAA-mPEG550 and 50% (w/w) EAA-mPEG1000. These types of films displayed water vapor transmission rates as high as 1130 g/m 2 day in high humidity conditions (Fig. 2) In low humidity, at constant temperature (conditions acquired as reported elsewhere) the same membranes were much less permeable to moisture, with a value of WVTR more than three times lower. The adaptive behavior is further confirmed by the presence of an inflexion point at 78% relative humidity. Figure 2 also shows a comparison between our system and other two commercially available membranes. Our system is superior, in terms of moisture permeation, than polyurethane-based monolithic membranes; as expected the WVTR is however lower than the one observed for microporous membranes without hydrophilic coatings.
40.0 µm
EMPA_SG 1.0 kV 7.0 mm x 1.20 k SE(M)
various lengths. The hydrophilic component is expected to adaptively lower the glass transition temperature of the grafted polymeric chain as a function of the relative environmental humidity.
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Further research is currently aimed at increasing the adaptive behavior and on increasing the WVTR in high humidity conditions. Additional work is conducted on the optimization of the production process of the basic polymeric systems as to enable a costeffective industrial transfer.
rH [%]
Support: KTI
We have recently designed and produced a series of novel polymers as main components for monolithic, adaptive breathable membranes. An ethylene-acrylic acid (EAA) copolymer was modified by acid-catalyzed esterification with hydrophilic polymers such as monomethoxy-polyethylene glycol (mPEGn where n is the polymerization degree of the main PEG chain) or monomethoxy-polypropylene glycol (mPPGn) of 50
Links: www.empa.ch/abt271
Contact: rene.rossi@empa.ch References: D.Crespy, R.M. Rossi, AATCC Review, accepted (2009)