A variety of moisture-vapor-permeable films for medical and garment applications are now available from Deerfield Urethane, South Deerfield, Mass. The thermoplastic-elastomer (TPE) films comfortably serve as moisture-vapor-transmission (MVT) membranes. They block dust, wind, and rain as well as blood, microbes, and viruses, but let moisture vapor, caused by perspiration, escape.
MVT films protect humans from the environment, and the environment from humans. For example, in clean rooms or operating rooms, they shield sensitive electronics from flaking skin or protect patients from microorganisms released by the gown wearer.
Deerfield, along with its German sister company Epurex, developed a range of breathable, monolithic TPE films based on polyether, soft-segment chemistries. Various grades of Pebatex, Walotex, and Dureflex films have amide (PEBA), urethane (TPU), or ester (PEE) hard segments. The TPEs are extruded into self-supporting films designed specifically for textile fabric laminates.
MONOLITHIC VERSUS MICROPOROUS
Monolithic membranes are dense, pinhole-free polymer films that transport moisture vapor via activated diffusion. They are produced by casting or extruding a sold film which is laminated to a fabric. Monolithic membranes can also be made from a variety of polymers and generally cost more than their microporous counterparts. That's because specialty polymer systems such as polyether softsegment TPEs are needed to improve performance and comfort.
Microporous membranes, on the other hand, are produced by stretching a blend of two incompatible materials such as a polymer filled with inorganic particles. Microcavities develop around the filler particles when the compound is stretched. These membranes can be made from a wide array of polymers from polyolefins to PTFE (polytetrafluoroethylene).
Monolithic TPE films for clothing laminates are not as susceptible to surface contamination and other related problems observed with some microporous structures. They are also more resistant to abrasion and other mechanical challenges. The presence of the membranes should also not be noticeable in the appearance and handle of the fabric. And in this respect also the absence of noise and smell are relevant. To fulfill these requirements a film should be formulated with a low modulus and coefficient of friction value, and low permanent set after stain.
IT'S ALL IN THE CHEMISTRY
A number of properties affect the permeability or breathability of a polymer. Its hydrophilic nature, crystallinity, and filler content can all affect the polymer's ability to transmit water vapor. The preferred TPEs are multiblock copolymers of the (AB)n type, consisting of polyether soft segments and crystallizable hard segments.
TPE resins with PEBA, TPU, or PEE hard segments usually provide the highest available mechanical toughness, and are good candidates for lightweight films. Manipulating the TPE's molecular backbone or increasing its water-absorption properties improves permeability of the film.
Base polyether soft segments are polyethylene oxide (PEO or PEG), polyethylene oxide/ polypropylene oxides (PEO/PPO), block copolyethers, or polytetrahydrofuran (PTHF or PTMEG). Plain PPO soft segments are, however, not typically found in breathable membrane TPE.
Blending different soft segments helps balance film properties. PEO, for example, is more hydrophilic, while PTHF is mechanically tougher and does not swell much. However, increasing CH2/O ratios in the soft segment decreases hard and soft segment compatibility within the PEO.
TPEs offer among the highest measured water permeability of all known polymers, in some cases exceeding those of hydrophilic polymers such as cellulose and polyvinylalcohol. The high permeability is related to their affinity to water. The downside of PEO soft segments is, however, a tacky feel and tendency to swell when heated during laundering.
The type of hard segment strongly determines melt temperatures. Nylon 12 and most TPU hard segments provide melting temperatures up to 100°C, while melting points of some polyester based TPEs exceed 200°C. A high melt temperature gives thermal stability during higher temperature processes such as lamination, seam sealing, and ironing. Mechanical properties or the type of hard segment also influence abrasion resistance while the nitrogen content in nylon 12 or TPU hard segments influences film toughness.
Breathable TPE films are generally laminated to at least one substrate to permit handling and improve mechanical toughness. Suitable substrates determined by the final application include:
• Woven and knitted fabrics for garments and wound dressings
• Nonwoven fabrics for wound dressings, inserts/linings, and roof membranes
• Foams for upholstery
Choose a lamination process to ensure breathability of the final laminate. Established techniques include flame bonding, especially for upholstery, and adhesive (spun) web technology. But as the first technique requires a foam layer and the latter is limited to a small number of materials, other lamination processes such as discontinuous lamination of the adhesive should be considered.
Important parameters for the microclimate between skin and membrane are temperature and humidity. To maintain a constant core temperature and a thermodynamic equilibrium between microclimate and environment (as a definition of comfort) the heat generated by human activity should equal the heat dissipated to the environment. The main heat loss mechanism, especially during physical activity, is evaporation of water and the subsequent transport of water to the environment.
During physical activity humans evaporate as much as 800 gm of water/hr, which corresponds to a heat loss of about 1,800 kJ, or about 80% of the heat generated. Water vapor can be transported to the environment by convection through the regular garment openings and/or by diffusion through stagnant air layers in and between the fabric layers. It is also driven by the difference in partial water vapor pressures of the assembly.