Skip to main content
main-content

Über dieses Buch

During the past 40 years many patents and articles have been published which describe methods and materials for water vapor permeable materials. These materials were primarily designed to substitute leather in its look and per­ formance. Later other industrial applications were found where microporosity or water vapor permeability could be used. The aim of this book is to give a survey of this matter, describing in an abridged way all the publications existing up to 1996. The terms "water vapor permeable material", "artifiGialleather", "synthetic leather", "leather substitute" or "man-made leather" are defined, and the special characteristics of leather are compared with its substitutes. Then the special methods used to produce microporous and hydrophilic materials, suitable substrates, end uses in fields other than wearing purposes, testing methods, as well as patent strategies and ecological behavior are discussed. Each chapter starts with general remarks about the the specific characteris­ tics of its motto; then published examples belonging to the subjects of the chap­ ter are described. Sources of most of the literature have been the "Textilbericht" and the of Bayer AG - today no longer in existence. Addi­ "Hochmolekularbericht" tionally, Chemical Abstracts and original publications in the form of patents and articles in books and journals are used. During the past 40 years many patents and articles have been published which describe methods and materials for water vapor permeable materials. These materials were primarily designed to substitute leather in its look and performance.

Inhaltsverzeichnis

Frontmatter

Leather and Artifical Leather

Frontmatter

Chapter 1. Leather

Even at rest, humans lose about 30 g of water an hour through their skin. Physical activity and sport can increase this to 1000 g an hour. We often wear garments to protect ourselves against wind, rain, heat and cold. The denser the fabric, the better it protects us against the elements. However, such fabrics tend to be uncomfortable because the water vapor released by the skin as perspiration cannot escape quickly enough. Mountaineers, walkers and skiers know all about this problem. Coats and jackets made of PVC imitation leather provide excellent protection against wind and rain but are virtually synonymous with extremely poor wear comfort.

Harro Träubel

Chapter 2. Coating of Textiles

Since the earliest times, man has used leather and textiles to protect himself against low and high temperature, rain, sun, wind etc. Due to their permeability to water vapor both materials stand for a high wearing comfort. Leather protects extremely well against wind, high and low temperature but is not easy to clean. When wet, leather is heavy and very uncomfortable.

Harro Träubel

Chapter 3. Definitions

DIN 16922 gives a definition of artificial leather as “textile and other substrates with or without a coating whose properties and/or surface appearance correspond to their use”. This — technically orientated — definition covers all man made, leather-like materials like homogeneous or foamed PVC coatings, polyurethane-coated textiles and poromerics [4]. In the main, man-made leathers are regarded as more sophisticated leather substitutes like coated nonwovens produced by a coagulation process.

Harro Träubel

Chapter 4. Water Vapor Permeability

Materials which are able to take in or let out moisture either need to be porous or selectively permeable. Membranes are products which are permeable to low molecular weight products or ions. In this work we will discuss mainly microporous or hydrophilic films, coatings and impregnations, i.e. materials permeable to water vapor. As will be seen later (Chap. 27) spin off materials, which were most probably originally designed as substitutes for leather, can be used for other purposes like filters, wound covers, slow release materials, etc. In general, all these kind of material are membranes — allowing defined products to permeate a barrier between an inside and an outside part of a technical product that they cover, include or contain.

Harro Träubel

Chapter 5. Microporous Products: Comparison of the Structure and Properties

Leather is structured as follows: Fibers of a coarser structure on the under layer, the reticular layer, become continuously finer and finer extending to the surface, the papillary layer, of leather [1] (Fig. 5–1). Extremely fine fibers build up the surface, i.e. the grain of the leather [3].

Harro Träubel

Ways To Create Water Vapor Permeability by Elimination of Solid or Liquid Products

Frontmatter

Chapter 6. Ways To Create Microporosity

Leaching of salts is the best known process for creating microporosity. This process to remove included products has been used for a long time: straw or salt is incorporated into a raw mass like clay. Then the clay is formed into bricks or plates and baked. Straw is eliminated by burning, salt must be leached. Another example is the production of diaphragms: Salt is incorporated into cement, then a plate is formed, hardened and afterwards the salt is leached. Diaphragms suitable for the electrolysis of sodium chloride to produce chlorine and sodium hydroxide are produced by this method [1]. In both cases articles with pores are created.

Harro Träubel

Chapter 7. Elimination of Solid Particles — Especially Leaching of Salts

A solid product is incorporated into a solution of a polymer. The incorporated solid substance must not be dissolved by the solvent used for the polymer. After impregnation, coating or film forming and drying the solid substance is extracted with a non solvent for the polymer [1–5, 13, 14]. At every place that the incorporated product has been, pores are created.

Harro Träubel

Chapter 8. Coagulation of Polyurethanes

The polymer most often used for the coagulation process is polyurethane. Nearly all leather substitutes sold in the world consist partially or totally of polyurethanes. Therefore a whole chapter is dedicated to the coagulation of polyurethanes.

Harro Träubel

Chapter 9. Microporosity by Evaporation of Volatile Products

If a polymer is dissolved in a volatile liquid and this liquid is removed by spraying, the polymer forms fine fibers which may be deposited on a releasing surface as a fine, microporous fiber fleece (see Sect. 9.1). In this case the molecular weight of the polymer should be rather low so that the polymer is able to dissolve in the volatile liquid. Another possibility of creating micropores is to mix a nonsolvent into the polymer solution in the volatile liquid. If this nonsolvent is also able to evaporate and has a lower volatility than the solvent, micropores are produced in a similar way to the coagulation process.

Harro Träubel

Chapter 10. Reactive Processes

All the processes described up to this point are based on high molecular polymers. In this chapter processes using oligomeric or low molecular “pre”-polymers are described. The polymers are polyadded or polymerized to a product of a high molecular weight during their application. A polycondensation which is less often published to produce microporous articles exists, however too [5].

Harro Träubel

Porosity by Other Means

Frontmatter

Chapter 11. Blowing and Foaming

Blowing is the liberation of gas which may create pores. Pores can only be obtained if the structure of the product wherein the gas is developed is strong enough to keep at least a part of the gas inside. To obtain porous products the gas creates pores in the blowing process. The gas is mostly liberated in the polymer during solidification to a film, coating or impregnation. Gas can be generated by adding a blowing agent like azoisobutyric acid dinitrile or azodicarbonamide (see Chap. 2.1) to the polymer(solution) prior to the solidifying step. By raising the temperature, the blowing agent liberates nitrogen gas [3]. Another product that liberates gas by raising the temperature is ammonium bicarbonate, which liberates carbon dioxide [8].

Harro Träubel

Chapter 12. Microporosity by Controlled Melting Processes

Glass filter frits are known to be produced by a controlled melting process. The melting is carried out in such a way that when a finely divided powder is heated up only the surface of the particles is melted. The particles flow together and adhere by their walls — they sinter. After cooling down, a porous membrane is created. If the particles are heated further, a homogeneous film is created.

Harro Träubel

Chapter 13. Perforation

Perforation by stitching or cutting can also result in a porous surface. This perforation should be carried out during a stressing action of a coated substrate [14, 15]. Apparently it is almost impossible to produce fine, invisible pores by such a standard perforation technique.

Harro Träubel

Chapter 14. Discontinuous Coatings — e.g. by Printing

Usually in textile coating an outer layer consisting of a hard polymer is produced during the coating process by a doctor’s knife application of a polymer solution or application. This process creates a homogeneous film. Poromeric surfaces are usually (top)coated by a finishing step, in particular to improve their abrasion resistance and to modify their color.

Harro Träubel

Chapter 15. Flocking

Velvet-like structures can be achieved by flocking. Thereby metal, wood or textile substrates are coated with an adhesive onto which the fibers are applied. The treatment of the surface of a substrate by fibers is carried out in an electrostatic field. Mostly small holes in the adhesive coat on the surface of the substrate result, thereby increasing its porosity. Flocking is carried out in the following way: a fabric, cardboard or leather is coated with an adhesive coat. Usually viscose fibers are either sprayed by air or in an electrostatic field [22] onto the coated surface. [9, 28]. Flocking [1] does not create eo ipso water vapor permeability.

Harro Träubel

Chapter 16. Crystallization of Homopolymers

Homopolymers consisting of crystalline and possibly amorphous parts can be transferred to microporous products. A homopolymer, like polyethylene for instance, is treated with a swelling agent like perchloroethylene. Then the foil is stretched and the swelling agent removed [3, 20]. Homopolymers like polypropylene or polytetrafiuoroethylene (PTFE) (9.3 [11]) are in a crystalline state at room temperature. If films are stretched a kind of mini-crack occurs, which remain as micropores. Polymers with a partial crystalline structure such as polyolefins, polyacetates or polyamides become microporous by stretching [21].

Harro Träubel

Chapter 17. Precipitation of Polymers in Water-Based Dispersions

The precipitation of dispersed monomer, oligomeric and polymeric compounds in water is easily achieved by changing the pH value of the dispersion or solution or by increasing the quantity of ions by adding a neutral salt.

Harro Träubel

Chapter 18. Hydrophilic Polymers

In nature hydrophilic products like protein, keratin or wool are responsible for water vapor permeability. The easiest way to obtain water vapor permeability for a chemist should be the of use a hydrophilic polymer. There are a lot of existing polymeric materials which are hydrophilic, e.g. materials of natural, semi-natural or of synthetic origin. Examples of those hydrophilic materials are proteins, cellulose, polyethylene glycol ethers, polyamides, polyacrylic amides, polyurethanes with polyethylene glycol ether soft segments, ethoxylated graft polymers, etc. Polymers which have a nonhydrophilic structure may be hydrophilized by mixing them physically or reacting them chemically with hydrophilic products or components. For example, ethoxy or polyacrylamide groups increase the hydrophilic properties of a polymer.

Harro Träubel

Chapter 19. Production of Synthetic Suede

Genuine suede leather is produced by finely buffing tanned and dyed leather from sheep or goats. Another source for suede is split leather from cattle hides which is produced by splitting the thick hides horizontally (see Fig. 20–2). Suede leather is mainly used in the production of garments. Genuine suede has excellent wearing behavior due to its high water vapor permeability. Its water repellence is not very good and its sensitivity to dirt is high. Genuine suede cannot be washed or dry cleaned without a loss of softness.

Harro Träubel

Chapter 20. Leather Board

Waste is produced during the manufacture of leather: Waste such as cuttings, leather dust and shavings. The amount of this waste may be around 3% of the weight of the leather produced. To reduce or correct the thickness of leather, the material is usually shaved with knife rolls, normally after the chrome tannage (see Fig. 1–8, step 11). These shavings can be treated with a latex and possibly further tanning agents on a paper-making machine to produce leather board. Leather board is a material which can be used as a lining material in shoes or trunks or for bookbinding. Leather board is called Lederfasermaterial or Lefa in German and cuir reconstitué in French.

Harro Träubel

Chapter 21. Nonwovens

According to a Japanese publication, the production of nonwovens has an annual growth rate of more than 10% in Japan. The growth rate for nonwovens based on polyester and polypropylene are responsible for this high growth rate, that for polyamide is stagnant. Polypropylene is cheap, resistant to chemicals and maybe used in the medical and hygiene fields [97].

Harro Träubel

Treatment of Man-Made Leather

Frontmatter

Chapter 22. Finishing and Dyeing

Leather receives a surface treatment in the finish to influence its optical and haptic effects. Leather is dyed (Fig. 1–8, step 14) in the wet phase of its production. Nevertheless dyestuffs and pigments are also applied in the finish to adjust or intensify the color of the material (Fig. 1–8, steps 18 and 19). The use of dyestuffs and pigments is primarily done to get a colored product and secondly to create special effects like two-tone, glossy or mat effects and so on. Leather in most cases needs a finish. The finish is used to protect the surface of the leather and to unify the way the different pieces look. Being a natural product leather very often has different grain damages or grain structures which do not allow leather pieces to be easily matched in one article.

Harro Träubel

Chapter 23. Modification of Physical Properties by Chemical Methods

The surface properties of leather substitutes often need to be improved in their water, solvent or abrasion resistance. Crosslinking is one of operations mostly used to modify a coating or an impregnation.

Harro Träubel

Chapter 24. Conversion of Water Vapor Permeable Products

There are processing techniques specific for leather substitutes. In contrast to leather, man-made leather can be processed in a continuous manner. Thermoplastic conditions are often used to form the surface to a grain pattern or gloss in a continuous process [14].

Harro Träubel

Chemistry, Testing Methods, Other Industrial Applications, Ecology

Frontmatter

Chapter 25. The Chemistry of Polyurethanes — Especially for Water Vapor Permeable Products

The author will not attempt to explain all of the literature about the chemistry of polyurethanes existing today (for summarized articles see [1]). Polyurethanes are not produced by a polymerization or a polycondensation but are built by a polyaddition reaction. Therefore they are called polyadducts.

Harro Träubel

Chapter 26. Testing Methods

When the development of leather substitutes was started the aim was to sub-stitute leather in shoe production. In the USA, Japan and Western Germany the water vapor permeability was characterized by measurements according to DIN 53 332, respectively, IUP 15 or similar methods [1]. According to IUP 15 a circular testing piece is placed into a small vessel containing a desiccant like silica gel. The testing piece is conditioned at 23°C and 60% relative humidity overnight. The next morning the silica gel is removed and replaced by fresh silica gel. After weighing the vessel, the absorbency of water vapor from the standarzied surrounding air is measured by weighing it after 8 h, The weight gain is divided by 8 and divided by the area of the testing vessel which has a constant size. The resulting value is a water vapor permeability measured as an absorption of water vapor at the silica gel in (mg/h · cm2). In Japanese publications the result was transformed into mg/24 h · m2. However, the results were comparable with the IUP 15 method.

Harro Träubel

Chapter 27. Other Industrial Applications

According to an analysis carried out by the Freedonia Group [1] in 1994 the US market for coated textiles was 294 Mio/m2. This had a value of $ 2.5 billion. Most was textiles coated with PVC for use in the field of transport. The sector of protective working clothing was 10% of this quantity. This market segment had the biggest growth rate. In view of this, PVC will keep its dominant role in the market [1]. Polyester will increase further its share of the market as a textile substrate. The combination of PVC as a coating on polyester as a substrate is not expensive and fulfills all requirements.

Harro Träubel

Chapter 28. Ecology

As we have previously seen leather substitutes in most cases consist of a textile substrate with a polymer coating and or impregnations. Therefore, the ecological behavior of the textiles is as important as the polymeric materials used. There are several environmental influences:

Harro Träubel

Trade Names, Marketing History, Summary of Patent Applications

Frontmatter

Chapter 29. Comparison of Different Articles

A comparison of the physical properties of leather substitutes using different manufacturing methods shows that these properties are similar regardless of the method used to obtain them: If the articles are manufactured by coagulation, selective evaporation or by stretching of a homopolymer, articles may be produced fulfilling all the necessary demands.

Harro Träubel

Chapter 30. The Development of Water Vapor Permeable Materials: Patents and Publications

The initiator in the development of microporous materials and the first supplier of a water vapor permeable leather substitute was DuPont (USA). DuPont, encouraged by the success of Nylon®, a synthetic substitute for silk, from the early 1950s started to develop a substitute for the last natural product, leather. Leather, a material which, up to that point, had not yet been reproduced in its characteristics by man.

Harro Träubel

Summary of Patent Applications and Practical Examples

Frontmatter

Chapter 31. Summary of the Patent Applications

Companies sometimes change their names; therefore, some of the names of identical companies are shown at the end of Table 31–1.Patent applications in most cases are characterized by the date of priority, i.e. the first application which is normally done in the country where the inventor lives or where the headquarters of a company are situated. The other dates belonging to a patent application, mentioned in the text, concern the date of application in the specific country and the date of publication which is important for a potential infringement of this patent. An infringement of a patent is only possible when the application is published regardless in whatever country.

Harro Träubel

Chapter 32. Some Practical Examples

In the examples given in Table 32-1, formulations are given which can be used for application directly onto a textile (direct coating) or in one or several layers onto a releasing agent such as a release paper (indirect process) and finally the textile is laminated into the polyurethane mass. The products selected are products of the LS or SP division of Bayer AG, D 51368 Leverkusen, Germany.

Harro Träubel

Backmatter

Weitere Informationen