High Moisture Vapor Transmission Thermoplastic Polyurethanes

Transcription

1 High Moisture Vapor Transmission Thermoplastic Polyurethanes Julie Samms, et al. Noveon, Inc Brecksville Rd. Cleveland, OH Introduction Thermoplastic polyurethane (TPU) has been meeting the needs of a worldwide marketplace for many years. Its versatile property mi has made it a candidate for many applications in various markets where a tough, durable, elastomeric material is required. These polymers offer design freedom not found in other materials due to a unique and broad-ranging portfolio of characteristics including: outstanding abrasion resistance, strength and toughness, chemical and hydrolysis resistance, fungal growth resistance, low temperature fleibility, clarity, colorability, ease of decoration, and a wide processing window in many plastic processes. These properties are so well established in the marketplace that there are very few market areas where TPU does not have some utility. Some of the segments that utilize TPU include: wire and cable, footwear, automotive, healthcare/medical, hose and tube, and film and sheet, as well as many, many others. A less well-known market for TPU is in the tetile industry where TPU is laminated onto a fabric to provide protection, comfort and aesthetics to the user or wearer. Noveon has developed breathable thermoplastic polyurethanes to meet the special needs of this segment. These polyether type high moisture vapor transmission TPUs can be etruded into thin monolithic films that can be laminated onto a tetile substrate. The TPU film then acts as a highly breathable barrier that allows moisture vapor to escape out from the body while preventing liquids from getting in. This breathability attribute of these particular TPU materials etend the use of these polymers into applications such as: sportswear, hygiene, outerwear, medical apparel, protective apparel, and others where it is important to balance comfort with an effective protective barrier between the user and his/her environment. The purpose of this paper is to discuss Noveon Inc. s new Estane (90A polyether) and Estane (80A polyether) materials which are now commercially available thermoplastic polyurethane compounds that ehibit the above attributes and have five to si times the moisture vapor transmission rates as standard TPU materials. Brief history of TPU in tetiles Thermoplastic polyurethane has been in use as a fabric coating material in the tetile industry for nearly 30 years. Its first application was as a replacement for PVC where the look and feel of PVC was of value but the performance of PVC was lacking in areas such as fle properties, plasticizer migration, abrasion resistance, low temperature properties, and cleanability. These deficiencies opened the door for TPU to replace PVC as TPU could solve these problems albeit at a higher price.

2 In addition to the above, TPU offered ecellent drape properties, softness and suppleness of hand, and could be processed to a very high gloss surface (wet look) or be formulated to look and feel like leather (shoes). TPU was also known to have much better moisture vapor transmission properties than PVC. Although this was not the prime motivating factor in TPU moving into tetile applications, it was a very desirable feature even then. Today, breathability of the tetile is of prime importance and TPU is still very much a solution to this problem. What is breathable and why is it important? Breathability is the ability of a tetile construction to allow water vapor to pass out from the body through it, but does not allow liquid from the outside to pass into it. This property is epressed as moisture vapor transmission rate or MVTR. It is the steady water vapor flow in unit time through unit area of body, normal to specific parallel surfaces, under specific conditions of temperature and humidity at each surface 1. The value is epressed in number of grams per square meter over a given period of time. ASTM Test Methods E96B and E96E were used to determine breathability of Estane and thermoplastic polyurethane. Protection is discussed in two different terms in today s tetile market. It can mean protecting the wearer from elements from the outside environment such as rain, blood or other liquids, virus, wind, and microbial attack. It can also mean protecting the outside environment from the human wearer in applications such as cleanroom apparel and surgical gowns. In the case of cleanroom apparel, the protection could be from particulate matter, such as skin, flaking onto the very sensitive electronics being worked on. In the case of surgical gowns, the protection is prevention of the contamination of the patient from microorganisms that could be released by the gown wearer. In 1991 the Occupational Safety and Health Administration issued a rule that requires all employers with personnel at risk of occupational blood eposure to provide appropriate protection that does not permit blood or other infectious material to pass through onto the healthcare workers skin or inner clothing 2. This rule has called into question the requirements for protective clothing in healthcare institutions. It has also been the impetus for a number of organizations, including ASTM, to issue test methods to evaluate the barrier effectiveness of protective apparel against body fluids and bloodborne viruses (ASTM ES 21, ASTM ES 22). The importance of comfort and protection in today s tetiles cannot be argued even though one can debate how much breathability is necessary for a tetile to be comfortable. Preventing water vapor produced by the body from escaping to the outside interferes with the basic chemical principle of achieving equilibrium. The challenge, then, is to create a film that is a barrier to liquids yet remains permeable to water vapor so that the fabric can relatively quickly reach equilibrium between the inside and the outside 3. Meeting this objective produces a tetile construction that achieves the goals of protection and comfort. Monolithic vs. Microporous Film Technology In constructions where a barrier film (such as the form in which TPU would be used) is employed to provide protection and also provide comfort, there are two technologies that are used to accomplish these objectives. Both technologies provide moisture vapor transmission permeability while simultaneously preventing liquids from passing through the film or coating. Which is better is a matter of degree of effectiveness. 1 American Society of Testing and Materials, Standard Test method for Determining Water Vapor Transmission of Materials Designation E96-95, P Krishnan, Hydrophilic Urethanes for Tetiles, Journal of Coated Fabrics, Vol 3, P Pinchard, Fous, Breathable Film: Now There Is a Choice, Insight 96 Absorbent Products Conference, 1996, Section XII - 2.

3 Microporous membranes provide a passage for gases through capillary-like pores (less than one mil and up) that inhibit the flow of liquid because of their small size 4. These structures are produced by stretching a blend of two incompatible partners, such as a polymer filled with inorganic particles. This stretching induces micro cavities around the filler particles 5. These membranes can be made out of any number of polymer systems, from polyolefins to PTFE. Figure 1 illustrates microporous membranes. Figure 1: Microporous Moisture Vapor Transmission Microporous Patent Art - Incompatible Polymer Stretching Interfacial Voids Gases and vapor permeate a microporous film by a variety of mechanisms depending on the size of the pores. Moisture vapor transmission rate through a microporous film should be directly proportional to film area and concentration driving forces. Physical structure of the film is the main determinant of permeability in microporous films 6. Monolithic membranes are dense, pinhole free polymer membranes that are produced by simply casting or etruding a solid film via one of several processing techniques and laminating it onto a fabric. These membranes can also be made out of a variety of polymer systems but are generally of higher cost because specialty polymer systems (like thermoplastic polyurethane) are needed to achieve the goals of protection and comfort to the degree needed. These polymers are also usually hydrophilic and absorb water very quickly. It is this important property that produces a wicking action that actively attracts water vapor. Figure 2 illustrates monolithic moisture vapor transmission. 4 Ward, White, Barrier films that breathe: How to make membranes comfortable around living tissue, Chemtec, November 1991, P Pinchard, Fous, Breathable Film: Now There Is a Choice, Insight 96 Absorbent Products Conference, 1996, Section XII Morton Breathable Thermoplastic Polyurethanes for Monolithic Membrane Films, Product Brochure by Morton International.

4 Figure 2: Monolithic Moisture Vapor Transmission Monolithic film allows the transmission of vapors through a process called activated diffusion. This is the same transport mechanism by which helium escapes from a toy balloon. The permeant dissolves on the surface of the membrane on the side of the highest concentration, and then diffuses across the film. When the vapor arrives at the opposite surface, the permeant desorbs and typically enters the surrounding airspace as a gas or vapor 7. Chemical structure of the polymer and film thickness are the main determinants of permeability in a monolithic film. Features of both technologies are listed in Figure 3 below: Figure 3: Characteristics of Monolithic and Microporous Film Technologies Monolithic Technology Microporous Technology Windproof Windproof Waterproof and liquid proof Water and liquid resistant Surfactant insensitive Surfactants cause leaks Selective permeability Non-selective permeability High water-entry pressure Low water-entry pressure Variable water swelling Little water swelling Ecellent tear strength Low tear strength Absolute microbe barrier? Ecellent odor barrier Poor odor barrier High MVTR High MVTR Moderate to high cost Low to high cost 7 Morton Breathable Thermoplastic Polyurethane for Monolithic Membrane Films, Product Brochure by Morton International.

5 Because there are no pores in monolithic films, they are generally the accepted technology in applications requiring a viral barrier. High MVTR Estane thermoplastic polyurethane technology is used to make monolithic protective films that ehibit the above characteristics as well as comfort. Thermoplastic Polyurethanes Offering characteristics of both rubbery elastomers and rigid plastics, thermoplastic polyurethanes (TPUs) have a number of features that make them desirable for use in a variety of applications. Their versatility and durability offer strength and toughness that is greater than that of traditional thermoset systems. They are also fleible enough to be processed on a broad range of thermoplastic equipment. TPU materials offer a wide range of properties. High tensile strength and elongation combined with superior resistance to tearing and cutting make TPU eceptionally tough. With abrasion resistance that is better than nylon, rubber, polystyrene or high-density polyethylene, TPUs are suitable for the most demanding applications. Members of the TPU family offer good chemical resistance, UV resistance and hydrolysis resistance, ecellent low temperature fleibility; and ecellent fle fatigue properties. Characteristics such as clarity, colorability, and ease of decoration make TPUs ideal for applications where aesthetics are important. The materials are also sterilizable, and bondable to a number of different substrates. Finally, the recyclability of TPU satisfactorily addresses growing environmental concerns. Several characteristics make TPU especially suited for use in the tetile industry. Their fleibility, especially at low temperatures, yields a soft hand and quiet sound to articles manufactured with TPU. The materials maintain good barrier properties while providing high breathability, a combination of attributes required by the outdoor and performance sportswear industries. Resistance to fungal growth, washability, rubber-like elasticity, resistance to tearing and punctures, and the ability to be processed into fibers further add to TPUs utility in tetile manufacture. Some TPU materials also provide anti-static properties, which are desired in apparel worn in environments such as clean rooms. General TPU Chemistry Thermoplastic polyurethanes are part of the thermoplastic elastomer family, a group of polymers that can be formed and reformed by the application of heat and pressure. All TPUs are manufactured from three primary raw materials: an isocyanate, a macroglycol, and a chain etender. The choice of each of these components effects the structure of the polymer and thus influences its final physical properties. Other materials such as stabilizers, lubricants, fillers and pigments can also be used in TPU manufacture. Isocyanate: Isocyanate used to produce TPU can be either aliphatic (HMDI, IPDI) or aromatic (MDI, TDI). Whereas aliphatic isocyanates demonstrate better light stability, are soluble in mild solvents, are more resistant to weathering, and offer better abrasion resistance, the aromatic materials are lower-cost, more solventresistant, and yield urethanes with better tensile strength and recovery. Hydrolysis of the final polymer is somewhat more likely with the aromatic materials, but this characteristic is more greatly influenced by the choice of polyol. Typical isocyanates are shown in Figure 4. Macroglycol: Macroglycols react with the isocyanate to form the soft-segment backbone of the polymer, and give the material its amorphous characteristics. Higher molecular weight macroglycols yield a longer polymer backbone and more fleibility in the final product. Choice of macroglycol type (polyester, polyether, or polycaprolactone) influences chemical and hydrolysis resistance of final TPU. Whereas polyesters provide better UV-, oidation-, and chemical resistance, and typically yield better physical properties, polyethers provide hydrolytic stability and good low temperature properties. Polycaprolactones provide a combination of these characteristics but are more epensive and can be varied in chain length only 8. Typical macroglycols are illustrated in Figure 5. 8 Gasparrini, F. J., Formulation Requirements for Urethane Fabric Coatings, Urethanes in Coated Fabrics, Vol. 3, Technomic Publishing Company, Inc.. Lancaster, Pennsylvania, 1995, pp. 52.

6 Figure 4: Common Diisocyanates used in Polyurethane Manufacture OCN CH 2 NCO H 12 MDI isomers NCO CH 2 NCO IPDI OCN CH 2 NCO MDI p,p' isomer NCO TDI 2,4 and 2,6 isomers NCO Figure 5: Common macroglycols used in Polyurethane Manufacture HO CH 2 CH 2 O CH CH 2 O H y Polyethyleneoidepropyleneoide glycol HO CH 2 CH 2 CH 2 CH 2 O H Polytetramethyleneoideglycol (PTMEG) HO CH 2 CH 2 O CH 2 CH 2 OH Polyethyleneoideglycol (PEG) O O H O (CH 2 ) 5 C O R O C (CH 2 ) 5 O H Polycaprolactone (PCAP) O HO (CH 2 ) 6 O C O (CH 2 ) 6 O H Polyheamethylenecarbonateglycol Chain Etender: Chain etenders react with the isocyanate to form the actual urethane structure, or hard segment of the polymer. Chain etenders are typically either diols, which react with the isocyanate to form urethane, or diamines, which react with the isocyanate to form urea. The urethane and/or urea can further react with isocyanate to form biuret and allophonate structures, which are undesirable side reactions. A higher percentage of

7 chain etender, or a short-chain low molecular weight chain etender, in the formulation yields more urethane segments and a more crystalline polymer, thus providing strength and toughness to the finished product. Typical chain etenders are shown in Figure 6. Figure 6: Common Chain Etenders used in Polyurethane Manufacture HO CH 2 CH 2 CH 2 CH 2 OH 1,4-Butanediol HO CH 2 CH 2 O O CH 2 CH 2 OH Benzeneglycol HO CH 2 C CH 2 OH Neopentylglycol HO CH 2 CH 2 OH Ethyleneglycol HO CH 2 CH 2 OH 1,4-Cycloheanedimethanol The Effect of Chemistry on Breathability There are a number of properties that affect the permeability or breathability of a polymer. Its hydrophilic nature, glass transition temperature (Tg), crystallinity, and filler content can all effect the ability of the material to transmit water vapor. The chemical structure of polyurethane can be manipulated to improve permeability. Incorporation of a hydrophilic backbone into the urethane including the use of hydrophilic structures such as ether, hydroyl, and carboyl groups increases the affinity of the polymer to water and improves permeability. Additionally, greater solubility of water in the polymer also improves permeability. The greater potential concentration gradient provides a stronger driving force for moisture vapor transmission, thus creating a wicking action in the material. An amorphous polymer ehibits higher permeability when in a rubbery state at temperatures above its glass transition temperature (Tg) than when in a glassy state at temperatures below its Tg. In a polymer containing both hard and soft segments (including most TPUs), permeation occurs primarily through the soft (rubbery) segment rather than through the hard (crystalline) segment. As a result, permeability is inversely proportional to crystallinity. Therefore, a more crystalline polymer is less permeable, and a polymer with lower urethane content/hard segment will have higher MVT properties. Permeability can also be effected by the addition of insoluble inorganic fillers. Of low permeability themselves, these materials act to increase the diffusion path length of water through the polyurethane and thus reduce the rate of transport. 9 9 Ward, Robert S. and White, Kathleen A., Barrier films that breathe, CHEMTECH, November 1991, pp

8 TPU s Formulated for High Moisture Vapor Transmission Characteristics: Estane and Estane Two TPUs specifically formulated for high moisture vapor transmission characteristics are Noveon Inc. s Estane and Estane These aromatic materials perform very favorably and economically when compared to traditional TPUs and to other TPE materials. Estane polyether-based polyurethane has a Shore hardness of 90A, tensile strength of 7300 psi, ultimate elongation of 500% and tear strength of 480 psi. It ehibits a moisture vapor transmission rate of 9,000 g/m 2 /day at 38 C/90%RH (E-96E). Estane TPU, a somewhat softer polyether-based material, offers better breathability. With a Shore hardness of 80A, tensile strength of 4000 psi, ultimate elongation of 800% and tear strength of 390 psi, it ehibits a moisture vapor transmission rate of 14,000 g/m 2 /day at 38 C/90%RH (E-96E). A summary of physical properties and breathability characteristics can be found in Figures 7 and 8 below. Figure 7: Typical Physical Properties of Estane and Property ASTM Method Estane Estane Hardness, Shore A D Specific Gravity D Tensile Strength, psi D Elongation, % D % Modulus, psi D % Modulus, psi D Graves Tear, psi D642C Taber Abrasion, mg D Figure 8: Moisture Vapor Transmission Data for Estane and mil etruded film Procedure Upright Cup, Water Upright Cup, Water Upright Cup, Desiccant Upright Cup, Desiccant ASTM Method Temp C Relative Humidity MVT g/m 2 /day MVT g/m 2 /day E-96B E-96B E-96E ,000 14,000 E-96E The moisture vapor transmission properties of Estanes and are much better than standard polyester- and polyether-based TPUs. Estane is comparable in both price and MVT performance to COPE and PEBA. The breathability of Estane is superior to that of these materials, and in fact approaches that of microporous PTFE. These comparisons are illustrated in Figure 9 below.

9 Figure 9: Relative MVT and Cost of Various Materials Relative MVT Relative Cost Estane 5714 COPE PEBA Estane Estane Microporous PTFE Processing of High MVTR Thermoplastic Polyurethanes Estane and can be converted into sub-mil film via standard thermoplastic processing equipment by a number of different means including: blown film etrusion, cast film etrusion, solution coating and calendering. Drying of these materials before processing is critical as they absorb moisture very quickly. A dehumidifying hopper dryer is recommended as standard equipment in processing these materials. Estane should be dried at 220 F for a minimum of two hours before use. Estane should be dried at 150 F, overnight if possible. Processors should be careful to avoid overheating the material as it may bridge or block at higher temperatures. When an etrusion method is used, a standard 24:1 L/D single stage screw is recommended with a 3:1 compression ratio. Initial recommended process conditions for etrusion conversion of Estane and are shown in Figure 10. These temperatures may need to be adjusted to accommodate process differences. Figure 10: Standard Etrusion Processing Conditions for Estanes and Conditions Barrel 1 temp., o F Barrel 2 temp., o F Barrel 3 temp., o F Die temp., o F Melt temp., o F

10 Markets and Applications for High MVTR Polyurethanes As described above there are numerous market and application areas within the tetile industry that can benefit from the use of high MVTR thermoplastic polyurethanes. These are shown in Figure 11: Figure 11: Typical Tetile Industry Applications for High MVTR TPU Application Medical/Surgical apparel Medical wound dressing Cleanroom apparel Sportswear, outerwear, footwear, gloves Military Clothing Why? Virus/microbial resistance, liquid-proof, drape, MVTR, puncture resistance, cleanability MVTR, elasticity and suppleness, bondability to other polymers MVTR, static dissipation, cleanability, non-gassing, nonflaking MVTR, low noise, low temp fle, fle properties, Wind-proof, cleanability MVTR, chemical resistance, low temp fle, Low noise, wind proof, fle properties Summary Highly breathable thermoplastic polyurethanes offer the traditional ecellent properties for which TPU s have become known. The materials can be made into monolithic films using conventional processing methods. In addition to good barrier properties, these films also provide high comfort and pleasing aesthetic qualities. This combination of characteristics makes Estanes and ideal material resources for the tetile industry.

11 Bibliography American Society of Testing and Materials, Standard Test Methods for Determining Water Vapor Transmission of Materials, Designation E96 95, 1995, Philadelphia, PA. Davis, D. G., and Conkey, J. B. New Hydrophilic Polyether-Ester-Amide Block Copolymers, 1991 Day, R. C. Rubber-like Thermoplastic Polyurethanes, Gasparrini, F. J., Formulation Requirements for Urethane Fabric Coatings, Urethanes in Coated Fabrics, Vol. 3, Technomic Publishing Company, Inc., Lancaster, Pennsylvania, 1995, pp Gasparrini, F. J., The Design of Solution Polyurethanes for Tetile Coating, Urethanes in Coated Fabrics Volume 2: Major Papers from The Journal of Coated Fabrics, 1979, pg First printed in Gillibrand, J., Polyurethane Coated Fabrics, Urethanes in Coated Fabrics Volume 2: Major Papers from The Journal of Coated Fabrics, 1979, pg First printed in Hayashi, S., and Giordano, C., High Moisture Permeability Polyurethane for Tetile Applications, Journal of Coated Fabrics, Volume 23-July 1993, pg Krishnan, K., Hydrophilic Urethanes for Tetiles, Journal of Coated Fabrics, Volume 23-July 1993, pg McCullough, E. A., Methods for determining the barrier efficacy of surgical gowns, Institute for Environmental Research, Kansas State University, Manhattan, Kansas, McCullough, E. A., and Schoenberger, L. K., Liquid Barrier and Thermal Properties of Surgical Gowns, Study done for Kansas State University Department of Clothing and Tetiles, Manhattan, Kansas. Pinchard, G. and Fous, G., Breathable Film: Now there is a choice, Insight 96 Absorbent Products Conference, Ward, Robert S. and White Kathleen A., Barrier films that breathe, CHEMTECH, November 1991, pp Estane Thermoplastic Polyurethane, Product Brochure by Noveon, Inc., Morton Breathable Thermoplastic Polyurethanes for Monolithic Membrane Films, Product Brochure by Morton International.